I've never used KJW so I can't compare, but I do own a WE. From my experience, they perform fantastic when they work, but somewhere along the way, you WILL experience some type of problem varying from minor to a little more PITA issues.
WE mags do leak like a bitch, but if you can reseal them (which is fairly easy), they'll be better than new. WE VN mags I haven't used yet, but will get some after hearing that they are very good.
One of the sweetest things about the WE system is that you can make it almost all out of real steel parts (modifications may apply) even down to little parts like pins. There's a lot of aftermarket support for the WE, you can use both airsoft or real parts.
For the KJW platform, I'm not sure if it's the same, maybe someone else can verify.