Of all the 6 button controllers for PCE none of them are very good for actually playing Street Fighter "for real" except for the Sigma stick (which is super hard to find) and the Hori sticks, which are pretty good but not as good as their new stuff.
Its unlikely that you are needing something tournament-grade though, and if you just want to play SFII' (which is unplayable with a standard pad) then, yeah, the Avenue 6 will do.