Get the Thule 591's (so long as they will suit your bike at the clamping point) if they don't fit then get some old Thule 510's second hand.
They are significantly better than the cheap Thule offerings and far far better than mounting on the back (perhaps with the exception of tow bar systems) in terms of ease of use IMO.
They will however cost you more in petrol as you are effectivley adding an air brake to your car, but when you are ready to go home or get on the road at the end of a ride you will be smiling as you put it on the car in 30 seconds (or at least that was my experience on a Scottish road trip)
Aero bars may be worthwhile if you intend to leave them on the car all of the time but otherwise the Thule square ones are fine.
Get it all cheaper off a internet based supplier, I think I have used skidrive.co.uk
If you have time consider hunting on Ebay for the car foot kit which is the expensive bit.
keppoch.