Would you consider either a trunk or hitch mount rather than roof mount?
We have used a Saris Bones 2 removable trunk rack on a sedan, and a Saris Cycle-On hitch rack on a sedan and a minivan.
The Saris Bones 2 (
http://www.saris.com/c-15-.aspx ) is an easily removable trunk mounted rack that would probably fit both of your cars. Check the Saris compatibility sheets. We have a Bones 2 that fit on both a sedan and a minivan. We've carried bikes several thousand miles with the Bones 2 with no problem. It has good sway protection to keep the bikes from bumping eachother or the car. It does take a few minutes to put the rack on the car and load the bikes. Also, I wouldn't leave the bikes on the rack overnight at a motel.
The easiest to use rack that we've ever had is the Saris Cycle-On hitch mounted rack (
http://www.saris.com/c-16-.aspx ). It comes in 2, 3 and 4 bike configurations. It takes just seconds to load the bikes and you only have to lift them 2 feet off the ground. Both of our cars now have hitches, and we move the 2 bike Cycle-On from one car to another in just a few minutes. It's even faster than changing the Bones 2. We've carried bikes cross country on the Cycle-On, too. The down side to a hitch rack is that, if the car doesn't already have a hitch, it will cost a couple hundred dollars to install one.