In terms of time it is much better to rent at TUS. All the rental companies are in the airport. I hate riding in the tram over to the rental car area in PHX. I suppose it depends some on your layover in PHX, but it will be much quicker for you just to rent in Tucson. I know the fares can vary quite a bit between PHX and TUS but it would have to be a very large difference for me to waste two hours on the 10 after wasting 45 min getting a car.