Rear, most definitely. Who wants to climb a top and risk damaging the roof/pana-roof.
Yakima or Thule are your best bet for a hitch mount rack. They do ones that allow the bike to hang from them as well as one like I have that is essentially a mini roof rack on a hitch mount frame which folds upright when the bikes are removed. I prefer roof racks myself but just bought a Cannondale Slash Carbon with a Lefty fork and it was too much a PITA to put on the roof with a tray that keeps both wheels on the bike. So I moved it all to my hitch mount which is hella easier to deal with imo.
Thule works great for hitch rack and roof rack. I have both on my Treg right now. Roof rack does not touch pano roof. I'll try to post pics tomorrow. I can carry 6-7 bikes with ease, maybe 8 with a little thought.
Just bought the Thule 916xtr. Mounts to the hitch. Easy to install, easy to remove. It mounts two bikes, and you can buy a two bike add-on. The thule folds up against the car when no bikes are on there. It can also drop down by pulling a latch that makes it drop towards the ground when you want to open the hatch and have bikes on the racks already.
Easy to mount the bikes as well. It has a ratcheting arm that hooks on to front wheel, and no parts touch the frame of the bike. It even has cable locks built in to keep the bikes safe.
The rack is heavy duty, and thus heavy, but it is very sturdy and well made.
Good luck with your search, I bought this rack after I saw all of the positive reviews, and agree with them.