Looking at a 2" lift kit for the rear of a truck. I've seen several aftermarket leaf additions to add the 2" and I've seen several 2" shackles. Price difference for either or is usually around a hundred dollars. Would an additional leaf make a stiffer ride compared to a shackle? When choosing between a shackle or adding a leaf which has the better performance in ride quality?
Leaf spring left kits usually have more leaves stacked and/or more of a bow in the spring. They are almost always significantly stiffer than the stock springs.
Shackles just lift the body up and away from the spring to give you more ride height and possibly more wheel clearance.
Which you go with depends on what you're trying to accomplish with the lift.