Concealment is a matter of practicality. What I mean by that is that you can probably conceal just about anything if you really want to go through the headache of doing it. So then it becomes really a matter of what is practical to conceal. Here's my opinion, it's worth what you paid for it:
The USP45 is a great handgun, but it is not really built for CCW. Too big, too thick, and just an all around hawg leg. On paper, just looking at dimensions, you wouldn't think so, but if you tried to conceal it you'd see what I mean. Some folks do get away with carrying it in an MTAC or similar holster, with some serious cover garments, but I don't think I could conceal mine to my own satisfaction in warm weather clothes, even with an untucked button down.
The Glock is another great handgun, but for me, it fails in the concealability department for the same reasons as the USP. Too big, wrong geometry.
There's really only one concealable option here, at least IMO, if you are talking about practicality...
Again, just my experiences, and my opinion. I'm sure there are folks that conceal either the G21 or USP45 to their satisfaction everyday, but it doesn't work for me, and I have what I would assume to be a pretty typical male body type (about 5'11", 170 lbs).
YMMV.
Jason