Basically, a combination of sharpness, noise, CA, etc. From a similar generation (e.g. 5DII vs. 7D), there's little, if any, difference. Cropping a FF image takes away most of the advantages of the larger sensor. But...if you can live with fewer MP (i.e. you're not making 30x40" prints), there's no downside to cropping the FF image, and a big upside if you don't need to crop.