Rachel,
The 50 1.4 is not really that sharp wide open! It's OK at 2.8 up, but on a 50D your not likely to get great sharpness at 1.4.
The 50 1.4 is pretty sharp wide open but needs a tad of chromatic aberration correction, although not as much as the 50 1.2 L at f1.2-1.4. In
general, Canon lenses are optimally sharp stopped down several stops from wide open.
Mike Shimwell said:
I suspect that the 5D is likely a better performer than the 50D in low light if you consider noise, but the autofocus is probably better on the 50D - not sure but it's certainly reported to be an improvement. Low light af is a difficulty. (Having said that, Canon offer higher isos on the 50D, so perhaps it's really a big step forwards... perhaps you should try both in a store - make sure you underexpose some high iso shots.)
Mike,
It's likely that with digic 4, the 50D will deal with noise better. with smaller pixels, the 50d will simply image the diffraction aberrations from f8.0 and smaller. I expect the 50D will be one of the finest cameras and even the diffraction issues will likely not be seen in most 8x10 prints. So it all depends on what Rachel's needs are.
The low light issue of the 5D is not so much noise, (which does occur, and can be calmed with Noise Ninja), rather it's the poor ability to achieve focus in low light* especially with the f4.0 lenses but even with the Canon 50mm 1.2L!
Asher
*Here's a trick. Get the person to hold a spoon by their face just to grab focus on that. After that is locked, the pictures from then in will be spot on. I also will focus on something like that spoon on the table a foot from the face and then step forward 1 ft to take my perfect picture.