Ok, my first and main reason is...i cant' actually name a Ritchie film, so I'm going with Tarantino first of all (there is an actual answer in that answer alone...if Ritchie was worth mentioning, there would have been a debate.)
However, i can hazard a guess at a Ritchie movie...did he do Lock Stock??? This is a superb movie (and I hope it was a Ritchie or else what follows is useless lol). BUT, it quite clearly takes infulence from Tarantino...so i'm not rating it.
Tarantino made the whole 'drug lord, cool guy, coke generation, heroin hitting messed up no-one' type of guy his main character, that the rest of the movie world just envied his creation.
Pulp Fictionn is the Bible of Cool Movie, and Ritchie can only bow before this bible...he will never achieve this God. lol
Ritchie made two great films, lock stock and snatch (the former being slightly better) and more recently rock n rolla which was a decent film. I would disagree with clairebear that Ritchie just copied Tarantino. The British setting and style made it quite different from Pulp. Tarantino didnt invent the cool drug using gun toting hard man character. There have been plenty of those in cinema before Pulp. But I would still go for Tarantino though. He has made many more decent films than Ritchie has.
I will have to say Tarantino. I don't know many Ritchie movies, but I have seen almost all Tarantino movies, and I must say, they are incredible. I specially liked Kill Bill, it was so.. dumb, but yet so incredible and deep. He is so different, and he isn't pretentious. The way he writes dialogue for his movies... most people didn't like death Proof because all of the talking and all that, but I thought it was incredibly fun to hear. I really admire the guy.