Sorry for the late reply, I thought I had notifications on but I guess not.
I have a differing opinion from TigerDragon on the mag versus drill mount. I recommend the NMO drill mount for most installs. If you plan to sell the vehicle you can just put an NMO cap on it and no one will notice, those that do generally won't care. I don't have a problem with mag mounts except they will most likely cause a rust ring on the roof. The magnet will pick up metallic particles, you will get road grime on the roof and the magnet will ever so slightly shift around acting like sandpaper on the roof. This will mostly be in the paint surface and not actually in the roof metal so it's going to be cosmetic "damage." A properly drilled and installed NMO mount will not leak or rust. If you go with the mag mount lay it down or remove it before running through low overheads as the damage from the mount itself can be more severe than the damage to the antenna of a drilled mount. The mag mount is an excellent choice for HT's or if you plan on moving it to different vehicles/locations.
As to what antenna to buy. I have have great results from the Larsen NMO 150 (5/8 wave 2m single band) and their NMO 2/70 (2m/70m dual band) open coil version. If I have to gripe on the Larsens it's the center pin in the base, it's a spring tab that sometimes binds when removed. Just make sure to check it when replacing the antenna. Having said that there are probably thousands of Larsen antennas being used today that are older than all the vehicles I have owned combined. I have had good results with Antennex/Laird antennas as well, and their center pins are rock solid. You will find that Comet, Larsen, and Laird will all be good antennas.
You asked so what kind of terrain are you working with? You will be somewhat limited by going dual band but that's not a big deal in reality. If you live in Mountainous terrain the 1/4 wave may work out best for you. The 1/2 wave is best suited for mag mounts as the vehicle ground plane, and the weaker coupling of mag mounts, is just added bonus. In the flat lands a 5/8 wave will be an excellent choice. That's all based on single band antennas though and the choices in a dual band are more limited. The Larsen NMO 2/70 is a center loaded, "shortened", 1/2 wave on 2m and two phased 5/8 waves on 70cm. It has a noticeably weaker signal on 2m but kicks ass on 440. 2m is still usable and not really lacking. Any multiband antenna is going to be a compromise on one of the bands but it's worth it if you need both of them. I don't use the 440 repeaters much but I do use the crossband repeat function alot and that alone outweighs the weaker signal by a ton.
Sorry if I rambled, its been a long day and my brain is taking a break.