Mason street, as far as I know, is actually not an HLE. It is a regular Hertz location located inside the city at a non-airport location. They do tend to have a larger vehicle selection and may have more specialty vehicles than a normal HLE location. The HLE location is actually about 2.5 blocks west on O'Farrell (550) and the prices can sometimes be lower, however they often times will be similar to Mason Street.

As stated above, if Mason Street does not have particular vehicle or selection you may try to exchange the vehicle at SFO where they generally have larger inventory. This won't guarantee a "better" vehicle and often times you may get what they have left, especially for a one-way rental to a different Own location.

If the prices are similar I'd suggest Mason Street over the HLE. I don't have much experience with the Avis, however my rental experience Mason Street was generally fine and I generally received a low mileage vehicle.