For $700, you can't go wrong with a 5DC. Like RL said, even an OLD full-frame body like the 5DC crushes a 50D and 7D in terms of ISO, color depth, contrast, and overall IQ.
In fact, Canon has made very little improvement in IQ to its current flagship crop body (7D) versus it's OLD crop bodies (10D/20D). According to DxO:
20D
Overall Score: 62
Color Depth: 21.9
Dynamic Range: 11 EV
ISO: 721
7D
Overall Score: 66
Color Depth: 22
Dynamic Range: 11.7 EV
ISO: 854
Now compare this to the 5DC:
Overall Score 71
Color Depth: 22.9
Dynamic Range: 11.1 EV
ISO: 1368
Having shot with all three bodies before (20D, 7D, 5DC), DxO's ratings translate to field performance rather well. Interestingly, although the 7D has the edge over the 5DC in DR on paper, I find that the 5DC handles fine tonal gradations (especially grays and blacks) soooooo much better than the 7D that I don't care at all about it's .6 EV disadvantage. If you can put up with its horrific AF and antiquated screen, the 5DC still offers exceptional IQ.