Either one would be a fine purchase there hasn't been any problems with the Gen4's that I've heard of and the one's that do have them all say it's a sweet shooter. The Gen4 G21 is the only Gen4 that I personally haven't shot yet. Plus for me I still think most of the problems are internet exaggerated.
It means be advised that I'm mean, nasty, and tired. I eat concertina wire and piss napalm and I can put a round through a flea's ass at 200 meters.