I used to use a roof rack, and due to car change (new one has a glass roof and didn’t fancy paying to repace it if I dropped a bike on it) I have now moved onto a towball mounted rack (thule 931? or something)
anyway, the roof bars are a pain in the erse ro put on and take off, so I used to end up leaving them on, not good for mpg. Pros – the bike goes up and secured in seconds (thule proride 591).
Towball mount – far quicker to fit the actual carrier, also, no reason to ever leave it on. It takes a bit more faff to secure the bikes though, especially if there is 3 of them, and even more so if they are big FS bikes. the carrier I have is supposed to drop down far enough to open the boot, but in reality, you’d need a boot like a mk1 mini to get it open once there are bikes on it.
Despite all those moans, I do actually prefer the towball mounted one!