I apologize for bumping this old topic, but I have done quite a bit of research on this, so I want to share my experience...
've used both cargo baskets and a couple different roof boxes, on a couple different vehicles. From my experience, the cargo basket makes more noise (empty) vs. roof box (empty or full, doesn't change wind resistance).
The only benefit that the cargo basket has is the ability to stack random-sized stuff -- duffel bags, cooler, camp chairs, etc. -- that may not fit into a roof box very well. A loaded cargo basket with random stuff tied to it has a lot of wind resistance, which turned into a larger impact on fuel mileage.
The cargo boxes require more careful packing to load them, but they keep the contents dry, secure (lockable box), and a lot less dirty (bugs, dust, etc).
Unless I really needed the space to carry a lot of huge, awkward, random stuff (i.e. like mounting a very large cargo rack across a big vehicle -- like a fullsize Con-ferr rack
https://wheelstips.com/best-hitch-cargo-carriers on top of a van), I would go with the largest rocketbox type cargo box that fits. I find them a lot more useful overall.
On YouTube there are a lot of interesting videos on this topic, I will share with you, one of them, hope this will help сомеоне.Good luck
https://www.youtube.com/watch?v=mRmZ4rsi7KM