Thule Freerides?
We've used Cruz "Bici" carriers for about 7 years - they're cheap, but quite agricultural, don't lock to the roof bars, and can be a faff to get bikes in and out of. If cost is a big factor, get these, and replace with more convenient ones as you feel the need. (But do be organised when mounting/removing the bikes (so as not to lose the locking units or wheel straps), and invest in a cable to go through the racks if you're leaving the bikes unattended in the racks).
In contrast, the Freeride locks to the roofbars, has nice QR type fixings. I also like the "A" frame design of the part that holds the downtube clamp, personally. The locks and wheel straps are integral (as for the rest of the listed racks) so it's hard to lose them!
The Proride is slightly easier to manage if you're struggling to reach the bike & fastenings, thanks to a clever fastening system that works from the level of the bars, more or less. My Dad has one of these - I don't like the way the clamp works top to bottom on the tube, but he's used it a lot and it's been fine.
We've also used the Atera Giro AF (on hire) and they were ok - I prefer the Thule Freeride personally though.
Both the Bici Carrier and Freeride clamp the sides of the downtube - the Proride and Atera Giro clamp them top to bottom (I used to put a strip of electrical tape behind the cables to stop them damaging the paint while clamped against the tube).
You can see all those carriers at roofbox.co.uk (it's where I bought mine from) -
http://www.roofbox.co.uk/bike-carriers/bike-carriers-racks-range.php?flt3=99&#filterform