i asked myselve the same question.
2 posts is verry easy because it takes in less room, and the plates are never in the way.
4 posts, drive your car up, and put it in the air, and most of these can lift alot more than the 2 posts.
just get yourselve some high stands, put them under your axles and drop the lift, you are now able to work around the wheels too..
I bought mysleve a 4 post, mostly because it can lift more, and with some cars (non chassis) you always have to be carefull where you put the stamps.