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The new iMacs released yesterday have some extremely impressive CPUs at the high end. The Sandy Bridge architecture in these iMacs, and the recent MacBook Pros, is so good that they’re competitive with the Mac Pro in some benchmarks.

So why buy a Mac Pro instead of an iMac or MacBook Pro?

I use a 2008 Mac Pro that’s still doing quite well, although now that its AppleCare has expired, I’ve started to think about what I’d get if an expensive component died and I needed to replace it.

Every Mac Pro revision after its introduction in 2006 has raised the prices of the midrange configurations. Mac Pros are now so expensive that almost nobody like me — geeks who like big, fast, expandable desktops but don’t do many long-running CPU-bound tasks, like video processing, for a living — can afford or justify them. Sure, I’ve gotten three solid years of use (so far) out of this one and it’s still doing fine, but it was also only $2800 for the mid-speed dual-socket model. (The similarly positioned model in today’s lineup is $5000 and is approximately 2.5 times as fast, which, while impressive, isn’t as far ahead as I’d like it to be for that price.)

In 2008, and for a long time before that, three major factors severely inhibited the performance and long-term usefulness of laptops (and iMacs, since they used many laptop-class components):

• Laptop hard drives were extremely slow relative to desktop drives, and most laptops can only hold one drive.
• Laptop CPUs usually had significantly lower performance than desktop CPUs.
• No laptop components, except RAM and the hard drive, can be upgraded after purchase. And RAM often maxed out too low for power users.

Some things have changed since then. For one thing, laptop CPUs are now awesome. But the biggest change, by far, is something that 2008-me never thought would be economical and practical: 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.¹ 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.

Last summer, I wrote a detailed post about why I prefer the Mac Pro to the iMac:

I have a Mac Pro and Tiff has a 24” iMac. … Now that both of our computers are nearly three years old, mine’s still doing fine for the foreseeable future, but we’re ready to throw Tiff’s out the window. …

While the Mac Pro costs a lot more up front, high-performance users also get a lot more value and versatility over its lifespan, which is likely to be much longer and end much more gracefully.

Some of these points have already become less relevant, but most are still true. The iMac still has very limited internal expansion compared to the Mac Pro. It’s still one integrated unit, so the display comes and goes with the computer. Most people still can’t (or won’t) upgrade or replace its hard drive or SSD because they’re so difficult and dangerous to access.

But I realized today that I’ve been thinking about the iMac the wrong way.

Since I’ve also kept buying laptops alongside the Mac Pro, the actual “cost” of the Mac Pro should include those. And I might not be doing so well there. Most of my “good value over time” arguments for the Mac Pro only apply if it’s your only computer, but for me (and many geeks like me), it’s not.²

Laptops have all of the same limitations as iMacs.³ We’re fine with that, because laptops provide a lot of value in other ways, and they cost a lot less than Mac Pros.

I should really think of the iMac, therefore, more like a laptop — but one that trades its portability and small size for a much larger screen and slightly better hardware. And that doesn’t sound like a very appealing tradeoff, since you can plug laptops into external monitors, and since many iMac owners will still want to own a laptop for portability.

It’s worth thinking about this if you’re considering an iMac purchase: will you still want a laptop? If so, will you be better served by just buying a fast laptop and connecting an external keyboard, mouse, and monitor to it at your desk?

My answer would be “absolutely”, so an iMac definitely isn’t right for me. The question for me, therefore, isn’t “Mac Pro or iMac?”, but rather, “Mac Pro and a laptop, or just a MacBook Pro?”

Two factors complicate this decision:

• The most desirable laptops for many people — the MacBook Airs — are too slow and low-capacity for many of the geeks who are still reading this post to use as their only computers.⁴
• The iPad might remove the need for a laptop, at least in some circumstances, for many people.

So, right as the MacBook Pro has become good enough to replace a Mac Pro for almost all of us and let us consolidate into one computer, Apple gives us good reasons to want either two or zero laptops.

I’m glad all of my computers are working, because trying to decide what to get right now would drive me crazy.