[Joe T]

I have owned both, but last garage I kitted out I went for a 2 Post, twin ram hydraulic model.
4 posters take up a bit more space and do restrict access to some areas of the car, even with twin jacking beams.
The spec on my last 2 post required 10" bolts into a reinforced concrete floor, as for stabiity I think you can make any ramp rock a bit, although the design of a four post gives it a natural advantage.

Make sure if you do get a 2 post that the ramp if any between the posts is low, and you get 2 stage pads for the ends of the arm so they can go low etc, a must for sports car work.

I have seen a car come off a two poster, and a 4 poster trapeze, I think you can break anything if you try hard enough, the 4 poster came down when someone put it drive with the engine running whilst at full height, the car managed to move a couple of feet into the endstop of the ramp, the inertia was enough. Although the ramp collapsed it didnt do that much damage to the car though....
Anyone whos spent time under ramps develops a six sense for getting out from under them if you are not happy.

I have done loads of engine changes on 2 posters, you consider this when first supporting the car as you move the relative centre etc with engine removal, also find them easier to get a crane under.

I think if you were new to a 2 Poster I would recommend a bit of training, not as straightforward as 4 poster.

I hope that helps.

Cheers

Joe T