Classic Army > G&G by a LONG shot. Externally CA are one of the best. THey're very hard to match, let alone beat.
Internally, it's a hell of a lot better than the G&G. Some claim there are problematic (inexpensive) internal parts like the piston body and tappet plates. But I haven't seen that myself. I'm running a CA M15 (same mechbox at that MP5) and finally blew my stock piston after 4 years of pretty heavy use.