I have tried (not on Omega) both roof-rack and rear carrier.

The roof bar bike mounts were strong and stable but were a pain to get the bikes on and off.

The rear carriers I have used are ok if they fit properly and very tightly fitted, with an additional number plate to stay within the law.  Only thing is, every one I have used has marked the car-one even dented the bootlid in under the weight of three bikes.  The bikes also overhang if you don't take a wheel off and restrict rear visibility.

Friends use a cheap but modified trailer with "dividers" which fits four bikes safely and easily-easy to load/unload too 

Cheers guys, kind of favouring the roof option, but not really looking forward to lifting them on and off.......