The short answer is this: if your priority is storing two cars in one spot, go four-post. If your priority is workin on the car, two post is better.
That's a really good summary. Just to put a slightly finer point on it: because four-posts lift the car by its wheels, to work on wheels brakes or suspension you have to make "special arrangements" Two posts don't have that problem but are sometimes not rated for long-term storage. This might be worth investigating further. In fact, if anyone knows of a 2-post that can be used to store a car I'd love to know; I'm in the market myself.
Also, one side note on 2-post lifts re working on non-running cars: the two posts have to "communicate" with each other. Typically this is a cable that runs between them. If that cable is at the bottom you either have to sink it into a ~1" trench in the concrete, or you live with a ~3/4" high metal cover running between the posts. Every time you want to raise an unpowered car you have to push it over that cover. That gets really old. So because of that I like the ones where the cable runs across the top which leaves the garage floor clear and flat.
And I guess 4-posts have a similar but not so easily solved problem: if you can't drive onto the ramps you have to push the car up onto them.
In either case, FWIW, Harbor Frieght makes a 120V-powered winch you can bolt to the floor at the back of the garage that comes in real handy for pulling dead cars in.
http://www.harborfreight.com/1500-lb-capacity-120-volt-ac-electric-winch-96127.html
Regarding motivation: aside from the safety issue already mentioned, I think the fundamental motivation is fatigue. Working under a car in a prone position has two fundamental problems: you expend a
tremendous amount of energy inserting and extricating yourself, and while under there your tool leverage and sight lines are severely compromised, especially if you are old enough to have problems with near vision (presbyopia aka "needs bifocals").