I have a 12,000 lb asymmetrical Rotary 2-post lift in my shop. Because the lifting posts are angled slightly, an asymmetrical lift gives you greater room to open a vehicle's doors than the other style . Other than that, the asymmetrical and non-asymmetrical lifts are the same.
The above comments are right on:
1) make certain that your concrete floor is deep enough and strong enough per the manufacturer's specs for the lift. There are lots of photos out there showing 2-post lifts that have collapsed, with catastrophic results.
2) get a name brand lift of a professional quality. Check out what the commercial shops in your area are using: BendPak, Rotary, etc.
3) I had mine installed by the Rotary distributor. It cost me $600. and they certified the installation.
4) If you are new to a 2-post, make certain that the car is positioned correctly on the lift front to rear for it's wheelbase. Also raise it to where the tires are just off the ground and shake the front/rear before you lift it all the way up to make certain that it doesn't bounce around. Better to come off the lift at 2" in the air than 6'.
5) On a 2-post lift, be VERY careful if you are removing weight from one end or the other of the car (e.g., engine, rear axle, gas tank, etc) when it is up in the air. ALWAYS use the tall (5-6') jack stands to support the car when doing this kind of work as the sudden weight shift can cause the car to come off the lift.
5) periodically check the floor for cracks around the lift. If it is starting to crack, then you probably don't have a strong enough mounting point for a 2-post lift and need to fix the floor.
Other thoughts on 2- vs 4-post lifts (my personal opinions only):
a) A 2-post lift is great for working on suspension systems, brakes, tires & wheels, lifting/placing a body on a frame, or removing/installing an engine cradle/subframe.
b) A 4-post lift is better (my opinion) for doing routine maintenance, such as oil changes, dropping transmissions/clutch replacements, driveshaft/u-joint replacements, setting ride height or drive shaft angles, etc. The reason I like the 4-post for these things is that the vehicle doesn't (shouldn't) move around when I am doing this kind of work under the vehicle; and the suspension is fully loaded when I am checking angles & ride height related issues.
A couple of other thoughts for consideration:
A) Don't get a "hobby" lift or those that are advertised primarily for double parking - they are not made as well as the commercial lifts and will not hold up well. Remember - it's your life you are risking - don't go cheap!
B) My shop is laid out and wired to have both a 2-post and a 4-post installed side by side for the reasons noted above. I only had the money to purchase one lift initially and that was a 2-post because it offered me the options to lift bodies for my street rod builds. The 4-post is a future purchase.
C) Spend some time looking at where to put a lift in your shop/garage. You don't want to block access to other areas/equipment and you also need to allow room around the lift.
D) Look at your lift needs. You don't need to buy a 12,000 lb capacity lift if you are only working on 5,000 lb vehicles. A 9,000 lb good quality lift will power up and down fine if you have the correct drive unit on it.
E) Get a 240V unit. The 110-115V lifts are really slow and struggle to lift a vehicle.
F) Look at the maximum lift height. I am 6'3" and needed a lift that would allow me to stand under the vehicle without being bent over. That means a pretty tall ceiling for me, but you can get other lower lifts that may meet your requirements. Make certain that you can stand up under the vehicle without crouching over. It will wear you out real quick if you can't stand up under the car. BTW, the lift doesn't need to lift the vehicle to 75" for me because the lifting height is to the bottom of the wheels/tires and there is 4-6" from there to the bottom of the engine/trans/etc.
G) make certain that the lift has a good, heavy duty interlock system. If the hydraulic system fails, this will keep the car from coming down. It is also a good idea to raise the vehicle to the desired working height and then lower it down to the closest interlocks so that they are supporting the weight rather than the hydraulic system.
These are just my opinions, and as usual, there are lots of other views out there. I really enjoy my lift and it has made working on my vehicles so much easier than crawling around on the floor or under a jacked up car.
Good luck.
John
Do I need a 2 Post Lift, or a 4 post Lift. 
7,000, 9,000, 12,000.
First thing I noticed, my ceiling height was 10 feet High. That sort of limited me, to a 4 Post Lift. 
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They take up More room, in a confined area, but I feel safer under it.
Do I want a longer unit, for a larger car? 
Many decisions. In a 4 post lift, the determining factor for me, was the way the locks engage the slots, in the posts. The cheaper lifts, have a ladder, that in bolted to the columns. Scary. You can rock, the whole lift. The only two companies, at the time (2016) Back Yard Buddy, and Advantage Lifts, had cut-outs, in the 4 Posts for stability. Almost impossible, for a car to come crashing down, once the locks are engaged into the slots. That was the most important reason, I bought an Advantage Lifts, model. Back Yard Buddy is made in the U.S.A but was more expensive. I'm not gonna go into the particulars, on this lift, because it will take too much time. They have a website, with instructions, how to get it off the Trailer, unpack it, and how to, put it together, after you buy it.
I am totally happy.
