Roof mount antennas are best because you get to use the vehicle as the ground plane, but on a MOG, you have height issues to be concerned with if you plan on taking it under trees etc...
Bumper mount is "ok"... But you want to make sure you compensate for what you'll be missing from a rooftop mounting. In particular, height and ground plane.
To fix some of the ground plane issues, you can get a Comet antenna, very stout strong antennas that are "ground plane neutral", meaning they don't require some huge flat metal area to get an excellent pattern.
In HAM antennas, height is everything, so get the tallest one you can get away with. One that fits your personal preference (Don't want you feeling it looks stupid!
) and functions well. There is a sacrifice here; looks vs. function, you give up one for the other.
So, in short... if you're roof mounting, get a good NMO mount and a LARSEN antenna.
If you're bumper mounting, choose a good mount (may also be NMO if you have a nice flat plate steel bumper to drill through) and a Comet antenna.