I have the same question. I see that Onion Valley is just 9200 feet vs 10,000 for Horseshoe. Wouldn't it be better to camp at the higher one to help with acclimization (all other things being equal)?
I was looking at the Mosquito Flat campground which is 10,300 elevation. I think you can hike to Mount Morgan from there, which I thought would be a fun prep hike after looking at Kathy/wingding's photos of her recent trip.
If any of these numbers are wrong, please correct me as I'm far from an expert on this stuff.