35mm is the out and out winner for street work IMHO. Technically less perfect than bigger formats but tonnes more good shots grabbed because it is smaller, faster, has more DOF, fewer film changes etc.

35mm can be effectively grain free at 20x16. Try any modern 100 speed film (Delta 100, Tmax 100, Acros etc) in dilute Xtol. D100 is great with xtol 1+1 and you will find grain only on inspection. Who cares though? I love some crisp grain with my street work and prefer films with more grain and grit.