Hi Ken,
I recently installed one - what are your constraints? as that will more likely determine the type you use.
If you have around 4m head clearance then you can use a 4t clear floor unit, that is the tops of the posts are tied together with a small beam and the hydraulic lines go across inside this beam. At this height you can fit a 4WD on it. This is what I have and it is great
Some have the option to lower this by around 250mm if you have less head clearance.
If you are struggling for height the the 2 post with a drive over hump is the next option. The posts are usually around 2.8m high and will give around 1800mm lift. The frustration of this type is you always have to drive over the hump.
There is another one similar to the hump type where the hydraulic power pack is portable and you run the lines across when you want to use the hoist. A fellow Club member just installed one of these as he has a low clearance problem and didn't like the hump option.
If you want more info please message me or email me (details on member profile).
Cheers,
Trevor_B