It sounds like you already have an investment in a rear carrier and roof bars. I'm not sure what you mean by getting the roof bars "fitted". If you have bars and towers and only need the vehicle-specific clips to fit them, that sounds like a good option. Then you can invest in one or two bike mounts for the roof rack and still use the rear mount rack. Roof racks work great as long as you can lift the bike on - tougher on some sport-utes or with a heavy bike.

One a separate note, I use a roof rack for our tandem and a single bike. I have noticed that with the rack and the bike on the car, I lose at least ~3-4 mpg. I'm only looking at the computer on the car and haven't really done a long trip and actually calculated mileage - something to think about with fuel around $4.00 gallon. I'm curious what others have seen and if anyone has actually done a more quantitative test.