My company mandates it, but one thing I find very nice is they have rental counters in many hotels, so its nicer than having to deal with airport returns and shuttles to the airport.

If I had my choice however, I would go with National. You pick your car from a lot of cars, so if you want a Saab and it's there, take it. Much more control over the type of car you get with National. Also, National recognizes frequent travelers much more than Hertz does and you accrue free days much faster and cheaper.