Shackles won't give you any more ground clearance than a body lift. Ground clearance is only changed by the size of the tire. Shackles and spacers will lift everything higher off of the axles, where a body lift will only lift the body. Shackels and spacers will gain you 1-2" max. For every 2" you add for shackles, you gain only 1 inch of lift - and spacers can't be much bigger than 1-1 1/2" as the studs aren't long enough for anything bigger than that. You will need an alignment ($40-60) after doing spacers - and maybe even new camber bushings (although not everyone needs the bushings). These run about $150 installed (depending on style. Yes - shackles and spacers are quicker to install, but the overall cost is much higher. Your best bet is to decide what you want to do long-term. If you plan on doing a full suspension lift at some point, dont do the shackles/spacers as you will be duplicating things down the road.
J