I also lean towards CA for its rock-solid externals. As for internals, you start with one of the strongest reinforced V2 mechbox shells I've seen to date and good, solid gears. The only items that are questionable would be the piston and piston head, both of which are cheap to replace.
I've got 50k+ rounds out of my CA M15 rifle so far. The only internals that have been swapped are the piston head / cylinder head (Systema silent), a Modify bearing spring guide, and Prometheus MS110SP spring. I finally swapped the hopup unit this past summer due to the stock one being loose and not holding its setting. The rest of the internals are all original and still going strong.
KWA are good guns too, but I have a hard time getting over those ugly-ass KWA trademarks on it. Trades are usually not a concern to me, but I want them either to be authentic, or have none at all. KWA's really bug me.