If you are looking for a dual-band antenna then the longer it is the better is will be, sort of. Longer meaning that there are probably more than a single radiating element on each bad, an array in other words. That only 'works' so far, then it gets sort of ridiculous.
A single antenna for each band will probably 'top out' as a 5/8 wave. They do work well and are still practical (such as when hitting things).
I've gone both routes, wish I still had the single band antennas. No really noticeable difference from the dualies with high gain claims, except in the wallet department.
I would recommend an 'NMO' type mount though. They are much stronger than the '239'/'259' types, are really water proof, and contrary to popular belief, that hole they require just ain't no big deal at all. Stay away from mag-mounts.
After that, which antenna you want is up to you...
- 'Doc