GHK M4 user here - well worth it and it is as good as advertised. Only downside is the mags, not just the cost but the weight. The mags are damn heavy and I'm used to running other gas systems. I literally have 10 of those and I only strap 5 on me for CQB and it gets heavy real quick when you have to squat, bend over, run e.t.c.
No experience with the TM/WE or VT, I do have a KWA MP7 - much more economical than M4. I only use it for indoor and it is reliable although the hop up needs to be replaced very shortly, and getting into that gun is much harder than taking down a M4.
Stay away from VFC GBB - I got a VFC 416C GBB, what garbage. At the end of the day if the mags suck the gun will suck. I got so sick of it I just stripped out the internals and replaced it with a daytona engine. Have not yet gamed with it yet, but once you fire a daytona gun in full auto it is a lot of fun.