Does anyone know what they think is the best value for rentals? Usually spend about $30-$50 tops on rentals, since there's the free upgrade.

I get 501 Avios currently per rental, but I'm wondering if I should instead switch to AA since I'll be traveling more with them next year (AA Platinium status through challenge)? I know they charge $1 per day, but give out around the same 500 AA miles. Or I could just hoard Avios until later? I've already reached the 4 rentals per year to get the extra 1,000 Avios.

I apparently have 633 Avis Preferred points, which I don't know how since you can't gain both at once. I know you get a free car at 700, is it worth it to switch to get to 700, then switch back?