Lots of choices, your answer lies in the intended use of the truck. Airbags are great for carrying a load, but they must be deflated and possibly removed for offroading (no articulation). Airshocks are similar but severly stress the shock mounts. The shock mounts were simply not designed to carry the load but are a quick, relatively inexpensive, fix. Shackles will provide lift but alters the arc the spring moves through during compression and can decrease lateral stability (important for towing large loads). Rearching and adding to the leaf pack gives lift, adds to carrying capacity and increases lift while maintaining lateral stability but is expensive.
This brings us to Add-A-Leafs and F-150 packs. All spring alternatives will sag over time, it just depends on the load you are carrying. Stock springs, stock load = sag. AAL, F-150, re-arch or adding to the pack = more capacity but if you use it they will sag over time too. If you simply add leafs without re-arching the new leafs they are carrying the load and fighting the stock springs "new" arch. They will flatten with time. F-150 packs replace all but the first leaf (which must remain the stock distance eye to eye and should really be re-arched to fix this). As a result, they will last the longer. Unless you overload them too.
Fully loaded for deep offroading, I carry 1500 lbs behind the front seats. 100k miles and 4 years and they are starting to get a little soft. Remember, within the realm of normal automotive use, all spring steel is the same. Capacity is related to arch and the number of leafs in the pack. Actually the distance between the primary spring eyes and the axle as well as this geometry plays a role too. For most people, the F-150 solution without re-arching is a reasonable one. It all depends on your use of the truck.