I've always used the in the window type, but they do leave the shiney spots that VIL spoke of, but I didn't worry much about it. I've mounted them in the back of the cab, along the size windows of an SUV, in the truck bed itself, and on the dog box. They're good for a lot of other things than guns too. Unbrellas, levels, binoculars, hats, coats, let your imagination go.
Back in the day when I drove soft top Jeeps, I used a bungie cord to strap the gun to the (padded) rollbar.
For God so loved the world, that He gave His only begotten Son, that whoever believes in Him should not perish, but have everlasting life.
John 3:16 (NKJV)