All good advice so far. Probably not much new to add, but maybe I'll say it a different way. We've stayed at both, but more often at the Riverside.
The Avi is miles from anything. So, when you stay there, it's to be AT the Avi only. The Riverside is right "in town", so since we're usually there for the casinos, that's where we stay. We don't tow, so we don't stay at Davis Dam.
We found the price to be about the same both places. (Which may make the KOA at Avi the cheapest KOA in the country!)
As for "casino camping" the Avi also has free dry camping available. "In town" we usually stay at the Pioneer. We often spend a week or so dry camping in the casino parking lots, then get a few nights in one of the RV parks to dump, do laundry, recharge batteries, etc.