I found an old thread from 2010 on this, but nothing recent. What are my chances of being upgraded to Premium class (from Full Size) as an Avis First member?

I have a Full Size car reserved with a discount coupon for $59 (all in) but I can reserve a Premium with a different coupon for $150 and be assured of getting a Premium. If I have a good chance of an upgrade, then I'd rather stick with the cheaper rental.

Also, what cars have others been getting in the Premium category?