Coronado Springs isn't typically a heavy resort... The only time I really ever have people standing on a Coronado Bus is during extreme peak times. However, being that you will be coming down when CBR is closed, and during a holiday season... it will be slightly busier than normal. All Theme Park busses go to Bus Stop # 2, 3, 4, and then #1. Downtown Disney busses start at # 1, then 2, 3, 4. The reason for this is so that guests can just hop any theme park bus to reach El Centro... and then a DTD bus to get back to thier stop.
Stops 2, 3, and 4 are never really crowded... Stop 1 is always the main and busy stop. So, I'd say go for Casitas (make sure it's near #2... some are closer to 1) or Ranchos (which is #3)
Stops 2 and 3 are rarely ever packed, in my experience... and since they're the first two stops, seats should be no problem.