Personally, if I had your suggested setup (21 x 9.5) on all four corners I would go with 275/35R21 all around.
Because I want the tire more flush with the outer rim lip, 255 vs 275 provides a slightly taller tire, overall more meaty. You will have more road noise, more flying debris kicked up by the tires (make sure you have front mud flaps), possible increased inner tire wear, slightly slower turn-in, and less understeer (front matches back vs staggered), but more grip.
Later I might go square setup, but for now I'm going with 275/35R21 on the 9.5" front and 295/35R21 on the 10.5" rear. I have the new tires already, getting them mounted soon.