Personally, between the two, I would go with the shackle lift.
A shackle lift will contain longer (and hopefully stronger) shackles with the kit to raise the rear end. They basically lift the truck from the frame to leaf spring.
An add-a-leaf suspension is an extra overload leaf added to your spring pack. Might make the suspension stiff all the way through its articulation. I have heard of cases of them breaking and being too rough on the (human) body.
NOW -- if you had another choice, you could get new springs altogther. I would check with companies like Deaver Springs in CA (I think Santa Ana, not sure) or National Spring in El Cajon, CA. I am told that Jeff at Deaver Spring is the better choice, as you can get your springs faster and he won't be happy until you are. A full set of springs for the rear will run you about $500. Not including shipping and installation. The springs can be set up for your type of driving (for instance, I do desert pre-running, I'll want softer rear springs) and for the amount of weight you normally carry in the back. I think that's a better option than to go with shackles OR add-a-leaf.
Hope that helps