I'll defer to all you guys that actually own them, but I will say that IME working on lifts, I've whacked my head far more on 4 post lifts.
Lifting on the chassis means that the actual lift arms are higher and out of the way much more for a given amount of vehicle lift.
IMO, this is the only real clearance difference that matters.
Not only does does the 2 post seem to have more head clearance, you quite often just don't even have to walk past the arms, so the ultimate clearance is irrelevant.
With the 2-post, the post lift is just out of the way much more so. I think they just work better if you are (like me) constantly going back and forth from vehicle to toolbox bench, etc.
I'm able to approach the work from the front or back of the vehicle and far less likely to whack my head.
Now, since I'm an idiot, I've still whacked my head plenty of times while using a two-post (typically a differential or shock mount hanging low).
That said, as a home gamer, I'd probably go four post.
Alas, it's all moot for me as my garage door arrangement means no lift for me at all unless I converted to a roll up door and that's probably as much as the cost of a basic 4 post.