Do you pack people or gear in your truck? Do you take it offroad?
From what I've seen, it seems most people get a little more lift at the front then the back, but it depends on your springs. Unless you really dig the low ass look like old 4Runners have, then you better put both in, because they will settle and if you put anything in the truck it will look really stupid. There's a thread here somewhere where someone has just the AAL's or just the shackles in and the rear is lower than the front. If you take your truck offroad, having both will benefit you in that it will lift your gas tank higher off the ground and also the spring hangers, both of which tend to get beaten up on stock trucks because they ride a bit low.
I like having the nose down a bit; it makes the truck look more aggressive and allows me to load up gear and people without having 4Runneritus. Plus, I haven't hit the gas tank on anything since putting the lift in.
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ChuckH
"Every morning when I wake up I know it's not going to get any better 'til I go back to sleep again!" Al Bundy