Both are as reliable as the other one. BUt, if size is not the issue, get the fullsize.
I used to have a fullsize 9mm USP, but the grip was rather big for me. I needed money for another gun purchase, so I sold it. I do have a USPc, though. And, the grip circumference is smaller on the compact. So, it fits my hands better.
But, while the grip is almost fullsize in length. the barrel is still a compact. I shoot it well, but not as well as I did the fullsize - especially at further distances.