I had a full-size USP/40. It was a very good gun. Easy to shoot, plenty accurate. I'd recommend it for a full-size gun in .40. I have no experience with the compact USPs. Hi-caps are out there for the full-size. I had them with mine, they were 13rd. I don't know if the extra three rounds would justify the extra expense nowdays though. The cheapest I've seen high-cap USP40F mags has been about $60 each, but at least you can get them.
I've owned a P226 in 9mm for a very long time and it's a great gun as well. I had a USP9F as well, but sold it and kept the Sig because I already had a couple dozen Sig hi-caps, and only a couple 10rd mags for the USP. I bought a hi-cap 9mm USP mag for $30 one time, but sold it for $60 on the other side of the gunshow later that day. I never had any problem with that USP either.
The HK is a big blocky gun (at least the full-size), but it's quick to use and lightweight. I like the way it was engineered and designed. I liked the way the safety system worked, letting you cock-n-lock, or hammer drop. I loved the mag release, as it was virtually the same as my P7.
The only reason I sold the .40 was to get out of that caliber entirely. I didn't really have a use for the gun, since I was carrying a P228 and had the P226 for a full-size. Now I'm thinking of getting a .40, and it will either be a HK/USP or a Springfield XD.
Ross