Didn't see this other wise would have replied.!! . . . . Just about every X3 I've seen use's 34's.
Was at some ones place other day who has the factory built X3 and that had 34's on it.!

Accel/Speed won't be problem with 34's for a mill and the extra torque would help when going gets tough.! . . .That said I wouldn't be worried using 23's either so I'd say either or.