We rented a mini van from Dollar in October 2004 from Dollar. It was new and clean. At the time of our trip our dd was 4 and ds was 1, and then there were 2 adults also in the van. We had a booster seat and car seat, plus luggage. I have to say that there was not as much room for the luggage as I would have liked. Not everything could be stowed in the cargo area. When there was no luggage and just the kids, us, and my parents in the van we were very comfortable.
I think that all of the on site rental car places are very easy to get to because all that you do is walk out of the terminal and into the parking garage.
As far as which company has the "best" vans, I think that they are pretty much all carrying the same kind of van - Dodge Caravan or Grand Caravan.
I don't know if this helps or not, but I thought I would offer up something for you.