Here are some very 'general' things about antennas.
The best place to put one is 'over' metal, not 'beside' metal. If you have to put an antenna 'next' to metal, keep it as far away as possible, and sticking above that metal as far as possible. The more metal 'next' to an antenna and not 'below' it the more detrimental it usually is. How much is too much? Good question, wish there was a simple good answer, but there isn't. What it amounts to is do what you can manage.
The most convenient is almost never the 'best'. A lot of it just depends on how 'convenient' it has to be in your particular circumstance/opinion.
One option, probably not the best in the world for several reasons, is to mount that antenna to the roll bars. Probably lots of "but's" to that, you'd know about them more than I would.
To some absolutely ridiculous degree, longer antennas are 'better' than shorter antennas. That's in both directions, by the way. A 1 foot antenna just ain't gonna be very good. Neither would a 20 something foot antenna. What can you 'live' with the easiest? The longer the thing is the more you WILL hit with it.
There is no -one- all around best antenna. There are antennas that are 'best' for particular circumstances. The 'trick' to that is figuring out which is which, and if you can put up with it's draw-backs (and they all have a draw-back of some kind).
The site for those Jeep mounts appears to be a good one! I don't have any experience with Jeeps so my opinion just doesn't count for much. There are those here who do have such experience and I'll bet they chime in. (Some times you gotta sort to 'poke' at'em, but they will. 
)
Have fun.
- 'Doc
