As has been said, the MT is slightly faster in climb, but slower in cruise where it counts for an airplane like the RV. It'll also move your c.g. aft and make it tougher to remove/install your lower cowling.
Some think that it looks better, but that's a poser argument and only valid on the ground. Do you care about that sort of thing? I sure don't.
Is all of that worth the extra cost? That's a value decision that each must make for himself. For me, on the RV-14(A), the Hartzell wins every time.