Honestly, I would suggest that you paddle any new boat a time or two before making a rudder decision.

That sounds very sensible, but it is more difficult to add the rudder tubes after the hull and deck are joined (for a wood core boat) or the frame is skinned (SOF). Not impossible, but awkward.

Also, the footpegs will be different for a rudder boat, so another (small) expense if you guess incorrectly.

The designer should be able to give you a definitive answer; I'd follow that advice.