Q: Why were self-propelled guns created in the first place? A: To better keep up with the tanks and to have less preparation time compared to towed guns.
Q: What would that mean for a wargame? A: Less Action Points (or equivalent) for movement and more Combat Rounds of fire.
Solution: Towed guns should take 1 Combat Round to properly deploy for an attack (or defense) after movement while self-propelled guns don't need it.
A round is 6 days of fighting. Artillery can be deployed in at most a few hours. It could perhaps get a penalty on the first round though. One possible advantage self-propelled artillery could have is being able to fire on units that broke through.