Pros: cheap, more arch, lift, hanlde more weight
Cons: more arch means your rear shackle angle gets reduced making it take more weight to kick it out to the rear. Not severe with just a couple of inches, but adds up quick. stiffer - less comfortable, less flex potentially possible choppy and loss of contact with the ground at speed.
Blocks are cheap but can quickly attribute to axle wrap
Shackles do add lift quickly but many times hasten the demise of your currently worn out springs.
I did a combination of xj-xj bastard pack and shackles to get what i feel was a good CHEAP compromise of added durability, lift, hauling, and flex. You have to weigh the pros and cons, read and ask ?s like you are doing, see what you feel comfortable with and then make your own descision. My answer or Joe Blows answer may or may not work for you and your situation.