I haven't bought from Willwood, but I have talked to him, and he makes great racks. I would go with him because a hitch system will be a lot more versatile than the Bird rack.
Also, some info on the Bird rack is that it permanently mounts two pipes under the car. Then you slide the bike, ski, etc. rack onto the two pipes under the car and lock them. Or at least that is my understanding of how it works. The mounts are not permanent, but I believe you must drill holes and they are intended to be left on. It might be a problem if your car is lower or with speed bumps, but it obciously works or they wouldn't sell it (or would they?).
Porscha
Threaded Mode
