Either will make a good gun. KWA's are very nice right out of the box. CA's are so-so internals
CA upper receiver pins break...seen more than a few with either one or both pins cracked out of the receiver. More due to user abuse during reassembly than wear/tear.
I haven't seen KWA tabs break...but I suppose they could just like any other metal part.
I do not like the "proprietary" KWA nozzle/hopup setup. The logo on the receiver is horrid. Out of the box several have had poor compression at the piston head. The solid aluminum piston head is not loctited and, in my opinion, not a good idea. I fixed one that had come loose...it destroyed the rest of the mechbox (the gears survived intact, the rest was toast). Rest of the mechbox parts are usually very nice.
CA's mechboxes tend to suffer from sloppy assembly. Piston is hit and miss. Should have a metal bearing spring guide. CA nozzles are hit and miss too. But with just a few upgrade parts and a little TLC, they can run a long, long, long time. I have not had good luck with CA Motors lasting long for anything but sub 350 setups. Some Modify SS cylinder heads do not fit (there is a CA specific version)
CA metal rifles are very nice. Painted on trades are a disappointment...but it usually takes quite a bit of wear/tear before they fade into oblivion. Solid builds usually. Bug juice is not good for AEGs.
CA's are just marginally easier to work on (really splitting hairs here).
If I were buying one (for the purpose of upgrading or not) I'd buy a CA. Regardless of what brand, I'd give the mechbox a once over anyways.