I would say let how often you need to tote a bike guide your choice.

For a few times a month, I'd put down a plastic tarp, and put the bike inside... with front wheel removed if need be. The plastic tarp will protect the carpet/interior from dirt, grease, and moisture, and also aid in sliding the bike in and out.

For a few times a week, or near daily use, by all means, get a rear mount or hitch mount rack.

I think the top is out of the question for anything but once in blue moon use.
JMHO.
-John