I'm in exactly the same boat as you right now - in the market for an imac for audio recording, sample-based work, etc.
You can actually get a 2011 27" imac for less than the 2010/macmall:
http://store.apple.com/ca/product/FC813LL/A?mco=MjMxMjQ0NDQ
For me, the 27" presents the best value proposition because you'd be getting a $800-$1000 display, and when you look at the specs with the remainder of the price, it is competitive or bests any other similarly spec'd all-in-one. Plus you can get 16Gbs of RAM at $130, which I think is the best price/GB ratio right now. To get more at a reasonable price you would need a mac pro, but THAT entails throwing down a ton more money in itself...
I figure you can install an SSD later if you wish - but Thunderbolt will also future-proof your storage needs. I think having the tbolt port is essential unless you wanted to hook up other things, like an xbox, to the display (which you can't do with thunderbolt, at least now as of this moment).
I'm actually pretty interested in what you have to say about this, because as mentioned, I'm in exactly the same dilemma.
Cheers
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So the only real trade off when you get an imac instead of a mac pro is the ability to upgrade, and the sheer amount of horsepower. But few applications and uses honestly take full advantage of 6+ cores or over 16GB ram, unless you're some kind of engineer, architect, or seriously committed gamer (in the latter case... get Windows).
I can imagine myself wanting to buy a new rig in five years anyway - and when that happens, I can still use the 27" as an extra monitor and/ or server.