I have used National and Dollar a lot in the past although recently I have been using National more since their prices have been competive and usually cheaper than Dollar's when I looked and I like getting to pick my own car. If the price is right with Dollar I would use them again. One thing to consider with Dollar, and this may have changed recently since I haven't used them in 2 years. The last time I used Dollar they consider the minivan class to be regular length wheelbases. If you want an extended length Minivan like the Grand Caravan as opposed to the Caravan they can charge you more at the counter for it. I have see numerous people here on the DIS who have gotten the Caravan for no upcharge, whereas others including myself were forced to pay the upcharge if you needed the larger van. They both seat 7 but the Caravan has almost no cargo space when all seven seats are in use as opposed to the Grand Caravan having some usuable cargo space. It seems to have been hit or miss. With National, I have always gotten an extended length wheelbase and there was never an upcharge. National even has Chevy Astro vans on occasion which are even slightly larger than the Grand Caravan or extended length Venture/Montana.
Good luck.