I use both, which gives me up to 6-bike capacity.
Quick bike on/off, don't care about interfering with opening the rear of the vehicle: rear mount.
Else: roof mount.
I actually have a complete Yakima round-rail system with 2.5 Raptor bike trays (2 work great, the third has a rusty bolt and is usable but annoying) that I was planning to craigslist once the weather turns nicer. I like the system; selling it because we sold that vehicle and bought one with aero factory roof rails (I bought the identical system for the aero rails). If you want it for, I dunno, $50 plus shipping (shipping will be pricey), let me know.
NB: bike racks destroy your MPG, even with no bikes on them. I always remove the rack/trays when not in use.