You're out of votes! What now?

The quality of instruction is the same at both in my experience. However Mountain House without question has the better setup. The check-in building is 2 or 3x larger and the crowds are 2 or 3x smaller. This equates to a MUCH less hectic and quicker and warmer experience for everyone.

The teaching area at mountain house is also much much larger and at the bottom of the mountain sheltered from wind and warmer.

The only reason to go to River Run is if your condo is there. I don't think Mountain House is enough better to go there if you're staying in River Run, but if you aren't, go to Mountain House.