I personally prefer them, but I'm a sucker for a quick change spring.
I've had zero failures in any of my or team mates Umarex/Ares guns, when run below 20bb/sec and below 400fps w/ .20g bbs. Our M60 is still rocking after many games of 10,000 bbs shot per game.
G36 vs M4 is personal preference. Try them both out, see which ergonomics work better for you.
I prefer the G36, personally, and HK in general. I just love paddle mag releases, among other small things.