A lot of virtual ink has already been spilt on the debate between owning an iMac and a Mac Pro. The issue has come up for me personally because I’m beginning to plan my next computer purchase and - like many others - I’m stuck between the two.  I’m not too concerned about power; The truth is that my 2006 iMac still delivers on all of my expectations. I don’t need a new computer so much as I just want to have something that’s the latest and greatest.

On this matter, Marco Arment has already elaborated exhaustively on a number of fine points :

On desk clutter:

My desk is clean and mostly free of cables and peripherals, but Tiff’s desk is covered in hard-drive enclosures. She’s using an X25-M SSD in a Firewire 800 enclosure as a boot drive, since the iMac’s internal hard drive is too slow. She’s using a pair of 1 TB disks in RAID-0 as primary storage, in another Firewire 800 enclosure daisy-chained to the SSD’s, because the iMac’s internal hard drive is too small. And she has another 2 TB external USB disk for Time Machine.

My Mac Pro has 4 internal hard-drive bays, so I don’t need any enclosures except for the occasional off-site backup disk. All of my disks are faster, quieter, and more reliable because they’re in directly connected, well-ventilated internal bays.

On pricing discrepancies:

The 4-core, 2.8 GHz base configuration above, at $3,873 (again, these prices all include 8 GB of RAM and the new 27” Cinema Display that you can’t actually order yet), is still going to be more expandable and versatile than an iMac. But at such a large premium over the Core i7 iMac, it’s tough to justify or recommend. It’s like buying an SLR if you’re only ever going to use the kit lens.

The Rise of SSDs:

The hard drive — usually the biggest bottleneck in personal computers, and formerly the biggest performance gap between laptops and desktops — can now be replaced at sane prices with an SSD that’s hundreds of times faster. The SSD is the most important performance increase to happen to personal computing in a very long time. And, notably, desktops and laptops use the same SSDs.

So the performance gap between desktops and laptops, and between Mac Pros and iMacs, has noticeably narrowed.

And the comment I’m most interested in:

Now that both of our computers are nearly three years old, mine’s still doing fine for the foreseeable future (although I’ll put an SSD in it soon), but we’re ready to throw Tiff’s out the window.

It isn’t particularly surprising to hear that people keep their Mac Pros longer than an iMac. Obviously, being able to expand on the capabilities of your computer can help infuse it with new life. But exactly how long can the average Mac Pro be viable in an industry that’s constantly improving and moving forward at a furious pace? Let’s crunch some numbers.

The following graph depicts the average Geekbench scores for every Mac Pro and iMac release since August 2006, when the Intel Mac Pro debuted, also covering the lifespan of the iMac since the Core 2 Duo processor was made part of the system. For each release date, I plotted the 64-bit Geekbench score for

• each baseline iMac/Mac Pro, the lowest processor speed available.
• each midrange model (sometimes stock configurations, sometimes BTO)
• each high end model. (Always BTO with the fastest processor available)

Using this graph, it’s easier to visualize the narrowing gap between the pro and consumer desktop lines Apple offers. It also let’s us compare the perfomance difference between new and old configurations of these computers, as well as the amount of time elapsed between each model refresh.

There have been 7 iMac refreshes in approximately 57 Months, or about every 8 Months. The Mac Pro has had 4 in the same span, or about 1 refresh every 14 months. Each update obviously improves upon the last but what’s interesting to note is the power creep present in the iMac line.  It might be hard to make out on the graph, but at each refresh, the baseline iMac comes close to or equals the Geekbench score of the previous generation’s mid level iMac. Before the introduction of the Core i5 processor in the October 2009 refresh, even the mid level iMacs were cannibalizing the previous generation high end models. The root cause of all this being Apple’s habit of recycling the same processors across the entire iMac line. Processors starting out as a BTO elite option tend to, over time, become the standard configuration for the low end model. The July 2010 27”  2.8 Ghz Quad-Core i5 iMac retailed for $1999, while the current baseline 27” iMac includes a 2.7 Ghz Quad Core i5 processor and costs only $1499. Similarly, the base 2010 iMac came with a 3.06 Ghz i3 dual core processor for $1199. At the same price, the current low end iMac has a 2.5 Ghz Quad-Core i5, a significant leap. Considered as an investment, one would be forgiven for getting some serious buyer’s remorse after shelling out for any low or mid range iMac, as the next refresh is significantly more powerful for the same price or cheaper. The lifespan of the low and midrange models is really just 8 months, a fact exasperated by the iMac’s limited expandability. Unless you’re under some serious budget constraints, it would be sound practice to always try and purchase the powerful iMac you can afford.  The current high end iMacs have more longevity. Even the first quad core option available in October 2009 still hasn’t been surpased by any midrange or low end option, the longest period in the computer’s history.

Which brings us back to the Mac Pro. If you purchased the base option in August 2006, you would have waited 3 years before seeing comparable technology on the iMac line. A mid range option purchased in July 2008 has only become outclassed by this year’s iMac refresh line. 3 years. If you purchased the first high end Mac Pro, you’d have waited nearly 5 years to see a comparable iMac. Even compared to other models in its own line, the Mac Pro is quite future proof. It wasn’t until December of 2009 that the baseline Mac Pro overtook the previous generation’s mid range model. Again, over 3 years. The mid range 2008 model has yet to be outclassed. In the case of Apple’s pro desktop line, purchasing a mid range model will protect you from power creep for the foreseeable future. In fact, prices have been steadily increasing across the line, helping maintain the value of older models.  While 3 years seems to be the trend, I’d argue that you’d get even more life from purchasing a top of the line Mac Pro today, where the performance gap between the highest end model and every other one has never been bigger. Even if the lower configurations get more powerful and cheaper in the future, it will be a long time before they catch up in terms of performance. Also, remember that the graph doesn’t take into consideration any upgrades you might make to your Mac Pro in the future. For instance, no Mac Pro ships with SSDs as a standard option. There’s lots of room for the Pro to grow.

So while there’s a much bigger upfront cost when purchasing a Mac Pro, that investment holds it’s value much longer over time. General consumers might not care much for this type of power creep, but for professionals and prosumers trying to stay on top of the latest technology, this is an important issue. Imagine an editing studio having to refresh their entire iMac laden setup every 8 months to be able to stay on top of the latest technology. It’d be disastrous. It wouldn’t be long before the tech guy would be fired for purchasing $1999 computers that can be had for only $1499 only months later. For companies following a budget, this is why the Mac Pro makes sense. Professionals want to be able to customize and adapt a computer system over a span of 3-5 years without feeling they overspent for it.

Even if you aren’t a professional, the Mac Pro may still be a good option. If you’re the type of person who actually does purchase a new iMac every 8 months to stay on the “power” end of the Mac line, why not spring for a Mac Pro? By purchasing a new high end model today (or whenever a refresh next arrives), it’s safe to say you’ll either come out even or below what you’d have to spend on similarly spec’d(Including ram, graphics and storage) iMacs over the same period of time.

Of course, not everyone has the $6000 upfront to spend on a top of the line Mac Pro. That’s precisely why the iMac line exists. Lucky for those people, Apple’s consumer desktop line has never offered more bang for the buck. If buyer’s remorse never affects you, you’ll be happy with your purchase for years to come.  If, however, you’re like me and are in a position to be able to consider either lines, it helps to know exactly what you’re getting for your money.