I’ll add my two-peneth worth.
For the record I used to have roof mounted carriers and now I have a thule tow bar mounted rack.
The roof mounted carriers were quicker to put on and off, just lift them up and a couple of clicks and they’re on and out of the way. The tow bar mounted one takes a little more care and attention.
There’s also the boot thing. My tow bar rack does have the tilt facility but its still not as easy as not having them there at all.
The fuel economy thing is a problem with roof mounted carriers though, well I say problem more of an issue. At the end of the day if you’re going out with a few mates then you share the fuel costs anyway. My last car (diesel Skoda Oct) did approx 50mpg combined, with the roof mounted carriers the car did 40mpg, still not bad. Tpw bar rack I haven’t noticed any difference.
With my tow bar rack i’ve noticed that the bikes get covered in road muck, something which the roof racks didn’t suffer from, not good if you don’t rinse or wash them when you get back.
In summation I probably prefer roof mounted carriers, but to be honest they’re both good and have there plus and bad points.