I won't comment on upgrades as I'm CHM and I'm sure your experience there will be different. But the Avis busses at DEN seem to always be quick to pickup - I've never waited more than a few minutes. And their lot seems to have plenty of cars every time I've been there (some week days, some weekends in the past ~year, including last week), so there doesn't seem to be a risk of no cars when you show up for you reservation. For a seldom-renter, I don't think there's any reason to avoid Avis in DEN.