Many people rent a completely different type of car then they are used to driving as sort of a long term test drive. Be very careful of this decision as a car that you are not comfortable with or used to is going to be harder to drive in a new or unfamiliar city or town.
To save gas and money, an economy car may be the best option for renting, especially if you are going to be doing a lot of driving. Most companies now offer unlimited mileage, but you still have to pay for gas. Keep in mind the bigger the car the lower the miles per gallon the vehicle will usually get.