For a 2 poster, concrete depth is only ONE thing as you also need to have the required PSI rating (strength) of the concrete required by the manufacturer also. Should be around 4000psi to 6000psi depending on the manufacturer.
There is also a couple different versions you can get, unlimited overhead but have cables you drive over (under a metal cover) and a cross over bar on top (that has the cables) between the two posts (open floor) with a stop switch connected to it.
The drawback on the plate/cable on the floor is if you roll your tool box around, only an inch or so high but rolling an 800#+ tool box over it would be a pain. I gave up rolling mine around years ago. On the overhead bar, you have an open floor but you may not be able to raise a small motorhome or similar height to the full chassis height. Just depends on your usage. Finding the lift points on a vehicle is fairly straight forward but finding the center balance is a calculated guess.
Fortunately I have a high roof. The things I looked for in the 2 poster is how high the bottom of the ARMs go and the width of the uprights. The bottom of the arms would be the lowest part of the vehicle, other than the tires, so I wanted to avoid hitting my head as much as possible. And the width was just that much more room for mirrors and getting in and out of the vehicles. The Asymmetrical ones (to me, again that's just me, as there are lots of them out there) just made me nervous on lifting anything at the upper extremes of the capacity.
I've had the Bendpac 9K# for around 15 years and it has worked for everything I've ever needed. Even used the arms as a lift to lift other non automotive items. I did buy a "stand" to use on my bugs for when I pull the motors out to counter balance it. Well, also the taller oil drain container when changing/draining oil, OMG I love that thing.
I have stored a couple of vehicles up in the air for a few months but in general try not to. Supposedly it's not suggested for long term storage on some manufacturers lifts but haven't seen it in writing myself.
As been mentioned about getting in and out of the vehicles, at least on my symmetrical one (arms are even side to side) is kind of a pain. I've gotten smarter, I get the wife to bring the car/truck in and out now.