It depends on the year in which you were born. For all the old guys like me (37) is Street Fighter the best beatn up ever. The movesystem, that Capcom allways use, is one of the best (easy to handel). The Characters like Akuma, Ryu, Ken, Chun Lee, are awsome. I Love Akuma.
Young people who may have witnessed the appearance of PS II, probably like Tekken more than SSF. Many young people played that game to see the high res movies that was in the game and saw them as a reward.
I like the fighting styles of tekken better than street fighter because all the characters' moves are way different. A lot of street fighter's basic moves seem too similar. Of course, i havent played SFIV so I'm not sure if they fixed that or not.