I've always gone with roof mounted - generally Atera bars (great quality, and sold assembled for your car, so none of that buy some bars and buy a foot kit malarkey) and initially Cruz Bici racks (~£20 ea, but a bit agricultural) then Thule Freerides (both framemount options). The roofbars get sold on each time we sell the car, and we usually get £40ish towards the next set that way, although our Astra bars, and Zafira bars went for £50. Remove the bars between trips to keep them in the best condition.
On trips where we only take one bike, we've often mounted a roofbox alongside - another advantage of roofbars if you need extra luggage capacity, and don't always need to carry a roof's worth of bikes.
We've looked at towbar mounted carriers from time to time, but have never been able to justify the extra cost (I can reach the roof to mount bikes ok, and we have no other need for a towbar).
