A minefield.
If low-light performance (both low-noise and AF) is an issue, I'd opt for the Nikon D700. But they're pricey (around $2500 I think).
But - It will use almost every Nikon lens ever made, so If you shoot a lot of portraits and landscapes, or macros, where AF is not really necessary, you can get bargain-priced top-notch glass.
Though I shoot Canon, I prefer Nikon's ergonomics and BQ but that's very personal.
If you are already invested in Canon, it would come down to a choice of FF vs APS for me.
The 5DII is a great 'general purpose' camera that takes speccy landscapes, portraits and macros, but the low FPS rate and 35mm sensor aren't for me.
As I take mostly long-ish shots and action images I opt for a cropped sensor for the extra length, and higher rate-of-fire, so I'd probably go the 60D.
Though it has more "features" than I really require and I'd rather have a few sensors that offer full cross-hair AF at slower apertures than more focus pionts that only work really well at f2.8.
It is however an improvement in image quality (at higher crops) over the 40D, but not hugely so over the 50. IMHO!
I can't think of anything really to recommend the 7D though I only had it for an hour or so.
Someone else here found the 550D 'noisy' though I haven't noticed that in daylight/flash shots.
I have not however seen any low-light images, so I stand ignorant.
Traveller's suggestiion to check-out dpreview.com is an excellent one, but stay away from the forums and stick to the test results.
I used to be moderator and members there are the most ignorant and opinionated SOBs on the web!
