KJW is MOSTLY TM compatible. I was looking into one a while back that had been upgraded with a bunch of aftermarket TM stuff. I remember someone told me what was and wasn't compatible, but I can't remember exactly what now.
Now, out of the box, the KWA is probably a better gun
But for all intents and purposes, you can easily find most replacement and upgrade parts fairly easily for a KJW whereas KWA parts will be harder to come by. They're also compatible with TM mags which are easy to come by. KWA are all proprietary and are compatible with nothing else.
Either is a good choice really. Depends on how much you want to spend initially and how much you want to upgrade.