If I stuck them on the outside I’d prefer to go tow bar for the fuel economy, secure fixing, and ability to drive into trail car parks with low bars without having the bikes flying off the roof when you forget. Not to mention rural roads with overgrown trees.
However, my much preferred choice is what I do, in the car.
Downside is the inside of the car gets scratched up and muddy.
The positives are it’s far more secure from both a safety and theft point of view, has none of the problems of being outside, and it’s not a whopping advert at popular trail centres saying “£thousands of bike kit here, follow me home / to service station and nick it at your leisure”.