I used to have a USP 9 Compact as well as the full size.
What I noticed immediately is that the compact has a plastic/polymer buffer instead of the second spring the full size pistol has. Also, shooting both pistols back to back, I feel the USP 9C is more, "snappy" and it gets back down onto target faster. That combined with the steel magazines, it became one of my favorite guns.
If your hands are small, you may prefer the Compact model more -- the grip is ever so slightly smaller in circumference than the full size.
That being said, I still prefer the full sized pistol (still own it). I like the longer sight radius as well as the larger pistol grip, both height and circumference. I haven't had issues carrying the full size concealed either. Your mileage may vary.