I would lean towards WE just for the fact it has a ton of aftermarket parts. Should anything in a KJW fail (please correct me if I'm wrong), you're pretty much limited to replacing it with KJW parts. I don't know, but if something is to break, I'd rather have the option of replacing it with a stronger aftermarket part than the same stock part that broke in the first place. If something in your WE fails, you can replace wiht aftermarket RA-Tech (or other) steel parts that will basically never break or wear.
Personally I haven't tried a KJW and can only speak for my own experience with my WE PDW that I used extensively throughout the summer with only 1 small failure to the nozzle spring, which was an easy fix. Otherwise it's performed flawlessly for tens of thousand rounds now with only regular cleaning as maintenance.
But I'd say overall both are excellent guns for the money, and you have to make your own choice which to go for.