The biggest capability differences between a 2-post and a 4-post are:
1. You can do suspension work easier on a 2-post, but you can solve that problem on a 4-post by using rolling jacks.
2. You can do a body lift with a 2-post but not a 4-post.
3. 2-posts are somewhat touchy about how the car is balanced on the lift and require paying attention to where the lift points are placed.
4. Removing heavy item, such as a trans, rear axle or an engine (if they come out from the bottom) can be tricky with a 2-post because of the change in the balance of the car on the lift. Tall lift jacks are mandatory.
5. 4-post ramps sometimes get in your way when you are working.
Other things to consider when buying:
A. Your height (at 6' 3" I needed a tall lift, but I had enough ceiling height to fit anything.) You don't want to be bent over when you are working - that gets very tiring.
B. Load capacity. I wouldn't get anything less than 9,000 lbs capacity. Although you may not need it for your current project, I can guarantee that someday you will want to do some work on your pickup truck.
C. Make certain that it is a good quality lift with mechanical interlocks over the entire height of the lift.
D. Some lifts have mechanical interlocks that can be released by a lever, some require compressed air to release the locks.
E. Unless you absolutely can't get 220V to your lift, get a 220V units. They move up MUCH quicker when loaded.
F. Stay away from the cheap lifts. Buy quality. You will never regret it. They are made better, last longer, require less service and may save your life!
Oh, and the last item. As you get older, you will REALLY appreciate the convenience of having a lift available, even if it is just to change tires!
Again, all of the above is just my opinion, but it is based upon experience.
John
My whole car, fits on the lift, with no hangovers. 
Best purchase, Ever. Hasn't missed a beat. Been working on M/C's and Cars, since I was 14.
I decided , I never wanted to lay on the ground Ever Again.
Other lifts have a ladder plate that's separate from the pillars. It is bolted, to the pillars. These types of lift's are less stable, and can wobble. I am never afraid being under my lift. 
When I got my lift, none of the holes lined up on one of the runways. I contacted them and told them I had to drill new ones to make it work. They were sorry and offered me 1/2 off one of those trolley jacks or a free floor jack. I was happy they did that over some mis-matched holes and just took the floor jack. BUT... 18 years later, I discovered the channels that are welded onto the sides of the runways for that trolley jack to ride in are in the wrong position on mine!! It's welded to the top of the runway and all I have seen, it's welded to the bottom of it. I never noticed it before because I never had the trolley jack. It was only recently I noticed it when I was thinking of building some type of jack. This of course would put that jack assembly a few inches higher that the vehicle would have to "clear" when driving on/off. I wonder how many got shipped out like mine???? Wish I spotted that in 2005!! Maybe they would have given me a full refund and keep the lift. (probably not)
