The DirectLift / Forward (same lifts under different brand names) units have adjustable lock ladders that will allow you to set the runway level even though your floor is sloped. The boxed column design with the lock ladders built in (like Advantage) will require you to shim the columns if you want your runways level.
Another feature I prefer to see on a 4-post is hidden/protected lock linkages. You'll see some brands leave the lock linkages out in the open, where they are more susceptible to damage.
After a lot of research, I went with a Forward EFP9, which is the same as the DirectLift Pro Park 9 Plus. One of the columns even has the DirectLift decal on it. After 3-1/2 years of very regular use, I still have zero complaints.
Whatever you go with, I recommend adding lighting to the runways. It's so nice not to need a headlamp or drop light.
And unlike a 2-post, you don't need a roll-around oil drain can. A simple platform for your typical drain pan works great and takes up less floor space when not in use.
Lastly, I'll also recommend picking up at least one bridge jack. With two, you can easily do tire rotations.