This is a really tough one. Both of these games are not only great to play but they changed the face of gaming forever.
Pac Man provided first real mascot in gaming. It's universal appeal helped open arcades (and, in turn, gaming in general) to a more broader audience. I could ask anyone and they'd know who Pac Man was but I doubt most would know who Ryu or Ken were. The gameplay is timeless and addicting, though at the same time can be boring and repetitive. The gameplay hasn't aged a day and is still fun and accessible. Still, Pac Man's relevance has diminished in the years since then.
Street Fighter II revolutionized an entire genre. It introduced character-specific fighting styles and special moves. Every fighting game to come out since owes it's very existence to this game. It has more replay value since you'd want to beat the game with each character to see their endings and, of course, play against friends. As important as Pac Man is, it's influence isn't as strong as SF II's, which is still felt with every fighting game released.
As for which game I'd rather play, it's Street Fighter all the way. While I like Pac Man, playing the same board over and over gets boring even with the increased difficulty. Street Fighter II never got old for me and is one of my most played video games ever. Hell, I still boot up Street Figher II now and again while I almost never play Pac Man. For that reason, Street Figher II gets my vote.