Which lift is preferred (2 post or 4 post)?

I'm focussing on your bad knees comment. Be aware, if the height of your garage does not allow a 4 post to go right to the top (with car on it) then you will be doing a lot of knee bending to get in under it to work the jack that lifts the wheels off of it. There are some pretty slick rods out there that you can use as an extension of your hand to set the arms of a two post in place rather than bending to a knee.