If you're concerned about luggage and that's pushing you to a bigger and more expensive vehicle, you can have one adult take a taxi with most of the luggage to the rental while the other adult drives the rental car, kids and a bag or two. Do the same on the return. That way you're not paying to rent the larger vehicle for the whole week. It will take some price comparison to see if that's worth it. I know people have posted some taxi fares on here as well as fare calculator links so a search should help you.