I imagine it's in part dictated by the teams you face. If you go against a lot of wiggs/chaze teams han's stun might be useful to delay biggs triple shot (stun him or wedge).
Leia however is really good if, as others said, you can get r2 to stealth her first. Probably not the best choice though if you fight a lot of aoe dispellers but her stealth makes her an optimal choice against counter attack teams