The new Apple iMacs look incredible. You can even get a quad core 27" one now. That beautiful screen and fast CPU with multiple cores is perfect for photography... right? Well, yes, but at some point, you'll probably wish you had sunk a little extra investment in a Mac Pro.
I bought a 24" 3.06GHz dual core iMac in late 2008 when I was shooting with a Canon 40D in full 10MPx RAW mode. The iMac handled it perfectly - it was awesome. Aperture 2 had no problem with those RAWs, and my nearly empty hard drive zipped along. I figured that two cores running at 3.06GHz should be enough for years ago come... But things started slowing down quickly, and there was little I could do about it.
In late 2009, I upgraded my camera to a Canon 5D Mark II. I now had 21MPx of RAW goodness, but of course that came with a price - slower processing and twice as much hard drive usage. I quickly filled up my iMac's "measly" 500GB drive, and since you can't upgrade the hard drive in an iMac, I had to go external. I bought a Drobo RAID enclosure and a couple of drives to start me out. The Drobo's great and offers data protection, but the housing alone is $300, and the file transfer speed went from 100MB/sec internal to 25MB/sec external. Believe me, you notice that your RAW files are coming across Firewire or USB, and it hurts.
In early 2009, Aperture 3 came out with brushes and other new features that I had been waiting for, but they too, came with a price - performance. I started waiting for brush strokes to catch up with my mouse, and got used to seeing the "Apple beachball". More RAM probably would have helped, but I was already maxed out at 4GB. By now, my Drobo had three drives instead of two. Needless to say, I was quickly outgrowing my iMac.
What made matters worse, because of my RAM situation, I had to close every app when running Aperture to eek out any little bit of performance I could. I would have had *no* problem shelling out $500 for 8GB of RAM at that point, but it wasn't an option - the box would have said "sorry buddy, I'm full".
When the 2010 Mac Pros came out, I went for it and got a 6-core 3.33GHz Mac Pro with 12GB of RAM and an extra slot for 16GB if I need it. If I wanna sell my car, I could go up to 32GB (it's too expensive now, but will come down by the time I "need" it). It's amazing. Since Aperture doesn't use all of the cores, it's not *that* much faster than the best performance I ever saw on the iMac, it's just that it *never* slows down and I can do anything else on the computer at the same time without ever seeing the spinning beachball. I've run two VMs along-side Aperture with no problem. I think you get the point. I'm now a little more future proof than before. There's plenty of hardware to spare, so by the time Aperture 4 or the 5D Mark II comes out, it shouldn't phase this machine a bit.
Also, with 5 available hard drive bays, I was able to turn off my Drobo and get back to 100MB/sec drive access. I even now have the option getting 2X, 3X, 4X, or 5X that performance by RAIDing the drives internally. Or, I could go nuts and get solid state drives which run up to 285MB/sec (as compared to a really fast spinning hard drive at 100MB/sec). All of these options come with different price tags, but they're only getting cheaper, and they're always available. I currently have 2TB of internal storage with plenty of room to grow. I can also upgrade my graphics card, monitor/display, and even add on components someday like Intel Light Peak and USB3 PCI cards. I can add on things that I don't even know about yet.
I also have the choice of displays with a Mac Pro. Many photographers are frustrated that Apple chose glossy for their displays instead of matte. Glossy screens basically become a mirror when you're viewing a dark scene or if your computer room is brightly lit. I just bought my Dell 27" U2711 and love it.
Don't Worry If You're Not A Nut
All of this craziness is because I refused to shoot JPG mode. If I had, the 2008 iMac would still be great. Someone out there is loving their new 24" iMac they just bought on eBay and questions why anyone would have sold it. If you're a sensible person, then just be happy with your JPGs, but if you're like me, and want the fully quality of RAW mode photography, then give some serious consideration to a Mac Pro over an iMac. I believe that I'll still be using this computer (with various upgrades) 4-5 years out, where I couldn't stand the iMac anymore after 2 years. The extra price of a Mac Pro becomes worth it when you realize how much longer you'll be using it.
If you shoot RAW mode and don't want to scream at your computer in a couple of years, then consider a Mac Pro over an iMac. You can still buy brand new 2009 model Mac Pros for cheap. Consider this one - an 8-core 2.26GHz for $2500, or this one - a 4-core 2.66GHZ for $1899.
If you're a serious photographer and you can convince yourself to go for it, I'm sure you'll be glad you bought the (more) future-proof Mac Pro over the sexy, but what-you-see-is-what-you-get iMac.