Jack's default is definitely down. Down is the first thing that he tries when we're shaping, and if I'm walking around with something awesome in my hand (my food, his food, a toy, whatever), he just hop-downs after me.
Missy's is definitely sit - and it's more like a butt-slamming, bouncy, SIT-SIT-SIT-SIT-LOOK-AT-ME-I'M-SITTING! sit.