I'd say to get the WE instead of the KJW if you're going for the Sig.
I had the KJW as my first pistol and although it worked great at first, it wasn't long before it was venting gas even on hot days. I ended up thinking it was a complete bag of wank.
The benefit of going with WE is that they're opening a distribution centre in the UK soon, meaning parts ought to be a doddle to come by, they're also very solid performers.
I have the WE Glock 17 Gen3 and it's the best pistol I've ever had. So I'd say WE again if you want a Glock.
As for USPs, I've no experience whatsoever with them, apart from having a go with a Match version by KWA for less than a mag's worth of ammo.
In terms of using them, again, I can't really comment on the USP, but for me the Sig is the most comfortable, it has perfect ergonomics, all the controls are within easy reach of your right hand thumb, you don't need to shift your grip at all to reach anything. If you have smaller hands then look at the E2 model, it has a part of the grip cut down.
The Glock is easier to aim though, in my opinion, the angle of the grip feels very natural, when I bring it up to aim, I'm already looking down the sights, whereas with the Sig, I find myself wanting to point it downwards a lot because of the grip's high angle.
Based on my experience, I can't recommend the WE Glocks highly enough. Being Glocks there are also a near limitless amount of after market parts, and being WE, they're TM compatible, meaning you can upgrade them about as much as most AEGs.
