I have a 4 post and like it. It has removable casters so I can roll it around the shop if I need to. Cost: If you buy a more expensive one, like a Bendpak, you get a quality unit that's well known and used by a lot of professionals. I bought a Direct Lift brand more than 15 years ago because of cost (didn't have deep enough pockets) and haven't had any issues, but it also doesn't get used every day, several times a day. Clearance: You want to determine the height of your ceiling (at the door opener or other obstructions on the ceiling) and the height of the tallest vehicle you expect to put on the hoist. If you get a two post, the cross bar can't be higher than the ceiling or anything that might be below the ceiling where the cross bar will be. Add at least another foot for clearance on either style. That's the highest you can go with your lift. Safety Issues: You want to make sure the locks are good quality. If you're under it and it falls, your day gets ugly in a hurry. Check for online reviews, other than on the manufacturer website. If you buy a 2 post, you'll want to cut the concrete under each post and pour reinforced footings. The size of them will be determined by the lift capacity of the hoist you get. If you buy a 4 post, I would recommend a 6" thick floor. Some lifts may say they work with a standard 4", but you're putting a lot of weight on 4 small pads, so you may crack the concrete.
PS. I've also seen several pictures, and one actual incident in a Costco service bay, of cars falling off two post lifts that had the arms placed incorrectly or the vehicle wasn't balanced correctly.