I've seen 'em both lots of times (annual cable TV "Godfather" marathons), to the point where I've gotten a bit sick of Mafia flicks.
I like II a lot more, but it's just personal taste and the original is certainly excellent. I guess I find the sequel much more wide-ranging and sweeping, with Michael's life following a sort of tragic trajectory. The original may be stronger photographically, but it seems more a straightforward narrative (especially since I read the book before the film was made) and less complex.
[Added] Though I prefer the sequel, I want to stress that the original is a classic film. IMO the "Sicilian Vespers" sequence, where Michael's godson's baptism is overlaid with rubouts of all Michael's rivals ("Michael Corleone, do you renounce Satan?") is one of the greatest in cinema.