I need to rent a minivan 8 to 10 times a year and usually keep the minivan for a whole week. Right now, I have to search for the lowest rate over all rental car companies. I can usually find a better price at Dollar, Avis or Alamo.

Should I stay within one company for my rental need? Do I save money in the long run?

How about National's Emerald Club? Can I upgrade to a minivan by just reserve a full size car all the time?

Thanks in advance.