Well, first of all, Mencia is Honduran and German. He was raised by his uncle's family but never took on his adopted family's name until he started the stand-up routines, probably because it made his brand of humor seem more authentic and it helped his career to look Latin and have a Mexican name. (FYI: I don't understand how anyone didn't catch on to the fact that he isn't Mexican, before I could even remember his name I'd seen his act, and I could tell he doesn't have any ethnic features of a Mexican at all)
I believe George has retained a lot of the culture and Mexican family values, whereas Mencia more or less pushes the fact that he was raised Mexican as fodder for his comedy act. It doesn't seem at all like he even wants to embrace that. And he sure as heck doesn't live the life.
I think George Lopez would be funnier than Mencia if I could understand everything he said and did. He definitely caters to the Latin audiences with his comedy, and you'll notice that depending on the venue he'll drift back and forth between English and Spanish. Since I don't know a lot of Spanish, I'll sometimes miss the joke. Mencia's style and routine are Americanized, and directed specifically at those who probably don't understand. Unfortunately I'm one of them, so I think Mencia is funnier.
But the only thing remotely Mexican about Mencia is that he can speak the language. Lopez keeps the culture with him, and uses it to inspire his act.