I only have personal experience with a KWA M93R II and a TM HiCapa, and got to talk with owner of, and try some, KJW, WE.
KWA has incredible gas efficiency, probably the best out there, with little cool down effect; 3 x 38 rounds Mags on full auto and it still had some gas remaining for a few more shots.
The nozzle (aka cylinder) is prone to break unless you maintain it, lube/clean really often. The return springs are a bit finicky too. Not too sure about the other series of KWA like the M9 PTP (I'd say they seem to be much more reliable, as the M93 has its share of issues if you look at KWA's forum)
Slides are metal, although light (a pro and a con, depending on what you prefer), polymer outer barel to reduce wear on the slide, which is fine. Gun is well balanced.
TM Hicapa seems to be much more reliable when it comes to the BBU, recoil spring/spring guide. There's Millions of parts available literally everywhere, which is a great plus.
Weight is not as well balanced as KWA's with no mag; inserting the mag mostly fix this.
Stock slide is plastic, but of great quality and can last for a good while. Aftermarket slides are heavier and sturdier.
Gun is much more accurate than my M93, especially when upgraded with a TK Twist Barrel
Good choice of sights and optics, the 4.3 has a rail. Trigger mechanism is much simpler and (I think) less prone to issues because of that.
Also, the Hicapa's frames is metal, unlike most if not all of their other models.
Having quickly handled a KJW (don't remember the model), I was not that impressed, but not disappointed either. Seems like a good gun, but I don't have much to say as I haven't looked at the internals.
WE, there are quite a bunch of horror stories out there, (they're full metal but apparently the frame is prone to cracking), despite the good comments on the PX4 on this thread. Note it's the first time I read comments that good about WE, but also the first time I read comments about the PX4 specifically. They're a lot cheaper than most other brands as well.
Hope this help a bit