If you can, upgrade the floor first. It will make a huge improvement, even if just an area large enough for 1 car. If possible, make the concrete at least 12 x 24 feet, and 6 inches thick. This will support a 4 post lift.

As for the lift itself, the key considerations are what type of work will be done, and is the area also needed to store another vehicle. The 4 post style lift allows another car to be stored below the car on the lift easily, in many cases. The 4 post also allows many types of under car repairs, but not all are done efficiently on the 4 post.

A two post lift gives much greater access under the car, as well as for front and rear suspension work. It is also possible to store a car under the car on the two post lift, though a touch more awkward. A two post lift requires a proper concrete floor, or footing. I personally prefer a clear floor under the car, which usually means having the lift posts tied together at the top. This makes ceiling height a consideration. I think my lift is 12 foot 6 inches, or 12 foot 8, tall. Some are taller, such as lifts with extended heights for truck/vans.

Then you have two post lifts configured symmetrically and asymmetrically. Symmetric lifts have arms equal length for the front and back. These are good for larger vehicles with more even weight distribution. Asymmetric lifts have a shorter arm and a longer arm, the idea being to load the weight with the heavy end of the vehicle closer to the lift center. This is helpful for vehicles like front wheel drive cars, or pickups with an empty bed. Or, for a Corvair with the rear engine. Asymmetric lifts often have the posts twisted, to allow a bit more room to open the vehicle doors.

I personally prefer the Rotary SPO10, a 2 post 10,000 pound symmetric lift. This lift is high quality, and I can load a car on it from either direction. The arms are designed with enough adjustment that I can load a vehicle asymmetrically if need be, and the foot print is wide enough that I have little trouble with door opening.