If you want an M9, only 2 companies make good ones, KJW and TM. As for which KJW model of M9, they are all the same on the inside, just different externals (which is up to you which one you like best). The mags on any gun will leak without proper care and maintenance.
The TM MK23 is very accurate, but HUGE. Take a USP and picture it about 30% bigger and you got a MK23. Its also made out of plastic, while durable, won't be as sturdy as a full metal gun.
Personally I'd avoid WE guns due to the failures I've seen.
Before you do take the leap into getting a gun, goto a game/ airsoft shop and fondle as many as you can. You might find that a 1911 works better for you than say a USP. Or maybe a 226 tickles your pickle in the right way.
As for Glock vs USP, I liked my RS USP CT more than my G17 when I had them. But I'd take a 1911 over both

(which is what I have now).