When two bases from the same brand cost the same, you take the stronger one, and that is the R21 Ultra. It gives you 21 Nm on Moza's newest flagship motor against the R16 V2's 16 Nm, and it scores higher on our rubric - all for the identical $699.
So the R16 V2 is really only the pick if you find it discounted below the Ultra, or you specifically prefer its slightly lighter output. Buying fresh at the same price, the R21 Ultra is more torque, a newer motor and a higher score for no extra money.