The are many reason to go full frame, but two of the most touted ones make me wonder:
1. Better dynamic range.
2. Lower noise.
The problem is that this argument is usually in the absense of the resolution, or generation. When I look at the numbers the Canon 20D and the 5D MkII have the exact same pixel size. So you can't talk about full frame having a 'deeper well' compared to a crop camera, or when comparing a crop camera to a compact. Not necessarily, anyways.
Personally, I find my 40D with smaller pixels offers lower noise than my 20D ever did. So what is going on? Why do the new 5D and 7D offer lower noise AND higher resolution? The answer is that there are several things going on.
1. More resolution means more noise
per pixel, but at the same time you have more pixels. So for a print of the same size, or view on screen at the same subject area you get to reduce on the higher resolution sensor. According to DXOMark this actually turns in the favor of the higher resolution sensor.
2. Each generation of sensor offers new technology. Larger microlenses mean that the actual pixel size gets closer to the actual area the light falls on. Better processing means manufacturers can increase the S/N to some degree. Thus, comparing a 5D MkII to the 20D isn't totally fair. Same size pixel
and new sensor technology.
3. Field of view - for a full frame sensor versus a crop sensor with the same angle of view you are gathering more light, onto a larger area, with more resolution. That helps...
So it isn't all hogwash, but don't tell me that a full frame sensor has larger pixels than a crop sensor or I'll pull out the cheap original digital rebel that has larger pixels than a 5D MkII. In case you are interested, the original Canon 5D and 1D MkII are still the champs of pixel size in the pseudo 35mm SLR sized camera market. That is probably why the Nikon D700 is so well regarded. They've kept the megapixels at the same size at the original 5D, with a few years in technology development.
