I am PC with Avis and EE with National. IME National is slightly better and slightly more consistent than Avis. I also tend to get better prices with National, though not every time. This probably has a lot to do with the code I'm able to use with National though. For upgrades I've historically done better with Avis, but it's still a bit random and location/timing dependent.

In terms of customer service they both can be hit or miss, with some locations being great and some terrible for both companies. TBH if you're getting better prices with Avis I would stick with them.