Dave,
I have a set of roof bars for my car and 4 bike carriers (bought incidentally from the Roofbox Co. and I too can vouch for their service. My wife has a tow bar mounted carrier for her car - a Thule Ride On 9403.
Therefore I am qualified to give an opinion on both!!
For ease of use I would say that the tow bar carrier is better. We bought it because there is no way my wife would have been able to lift hers and my 2 girls bikes on to the roof of her car.
It is actually much quicker to fit to the car, and once you get the hang of which way round to put the bikes on the rack it is fine. The rack can seem to bounce around a little on the move - which can be a little disconcerting when looking in the rear view mirror - but the bikes always seem to remain secure. It doesn't appear to affect the driving of the car either.
If we are leaving the car in a dodgy area, then it is a 2 minute job to take the carrier off and put it in to the boot. The 9403 is about £100 and includes the lighting board. All you have to buy extra is the number plate.
The Tow Bar was about £200 including the proper full electrics kit. I saved a bit of money on that as I fitted it myself.
As fossyant said, the roof mounted bars and carriers really affect fuel consumption and are noisy and also take much longer to fit. Overall it is the cheaper route depending on the specific carriers you buy. You can get them for £20 each or pay as much as £60 each. They probably hold the bikes a lot more secure but the height can be a problem - particularly getting in to public car parks which have the height barriers
Overall, I would recommend the tow bar mounted carrier.
Hope this helps.
Gary.