Do yourself a favor and do not use a mag mount.
First, properly installed mobile antennas work much better than "stuck-on" ones. One of the most popular is the Larsen NMO270, dual bander.
Secondly, mag mounts can be dangerous in the event of a crash. In February of this year, a bystander died as a result of being hit by a mag mounted VHF antenna. Personally, I wouldn't take the chance.