Are you buying from a dealership? If so, take the bike down there and try it if they'll let you. Otherwise, I'd just wait until you have the car before making a decision. You can always just carry the bike inside the car until you figure out the best solution.

You'll hear horror stories about all kinds of bike racks. My advice is to buy a high quality rack, and just buy the type that suits your needs best. I prefer roof mounts, but there are plenty of horrow stories about people having a brain fart and driving into a garage and doing a lot of damage to the bikes! With hitch mounts, there is sometimes a higher risk of scratching the frame, and the bikes tend to get dirtier. Swings and roundabouts...