You are going to be splitting hairs, really. The trailer will go through the air easier, but will add some drag due to frictional losses from the trailer's tires and axles, not to mention whatever the trailer weighs.
I would just go for what is easier/more convenient based on how often you are going to be using it.
I have had roof racks on my cars in the past, and found them a pain to take on and off... much easier to just lash a trailer to the car in my opinion. But, if where you are going makes dealing with a trailer a different kind of pain...
Just know that no matter what, your car will still net some pretty good fuel economy. My WORST tank ever in my 2000 Golf was 39 MPG.... and that was with a 1991 Jetta on a tow dolly behind me crossing the Appalachians.