I have used the USP series and I'm certain that they're all very nice regardless of which model you get exactly, however I've heard that the HK45 has some strange issues that are unique to its design. The MK series on the other hand I have lots of experience with and all I can say is... wow. I've seen WE's, WA's, TM's, KJW's and other 1911 replicas in person and I'm 100% percent honest when I say 'none of them compared to my KWA' and while I'm sure the TM could have been upgraded to a metal body or have better parts yada yada that doesn't mean much to me since you'd have to invest around $500.00 into it before it was 'done.' The capacity of the MK series used to be the only real downfall but with the new 21 round magazines you're really not missing much. I have the MK II and have been putting it through some fairly rough use for over 2 years now, still runs like day one. I'll admit that it had a couple issues here and there but I was easily able to fix them since the build design was so intuitive and simple. Range, accuracy, gas efficiency and durability are all top notch; the materials are tough, the design is easy to fix, the 14 round mags were able to shoot about 2-3 mags on one charge on a good day, accuracy will depend on your ammo choice but I've found that it's a very competitive worthy pistol.
All in all if you go with a KWA pistol you won't be sorry, so my suggestion to you is to pick whichever one is more comofrtable to you; which one can you hold at your side, close your eyes, bring the gun up to sight, then open your eyes and have the sights on target? Good luck with choosing!
Edit: My close buddy has the MK IV, there is no difference at all between our pistols in terms of performance. He likes the OD and extended guide rod, I liked the FDE with the straight barrel.
Edited by General Pickle, 21 July 2013 - 08:44 PM.