The mag mount depends on capacitve coupling to ground while the "drill a
whole in the roof" mount is directly connected to the ground plane.

Also, when using a hole in the roof antenna there is no problem withcoaxing
being crimped in a door or window, etc. I have used the NMO (Motorola
style) in the roof mounts for 2 meters and 450 MHz since 1978 with various
vehicles. No problems. IMHO, it is the ONLY way to go.

John