it depends on the type of terrain, distances involved etc. A 9db antenna will give you very good range but doesn't have a big radiation pattern... mainly for open flat terrain.
The 6db will give a bigger pattern than the 9db but loses range.
The 3db will give you a bigger pattern again but has a very limited range. This type of antenna is good for bush driving where you are relatively close to the other vehicles.
You have other antennas as well but the 4.5db antenna would be the main one used over the ones already mentioned.
Mounting position also has an effect on the radiation pattern. But for general comms, mounting the antenna on the bullbar gives good coverage all around the vehicle.
I've done a little sketch to highlight the radiating patterns... (I'm no artist as you will see)
The actual distances from a 9db to a 6db and a 3db would be in the vicinity of...
9db... 20km
6db... 12-14km
3db... 6-8km
These distances would be approximately the maximum for each of the antennas. Ranges could vary depending on how well the antenna is grounded and how good your radio is... etc etc. The pic doesn't really show this to any sort of scale... sorry.
There was a good article in 4wd action about a month or so ago... worth reading if your interested...
Cheers,
Mick