I believe that the adjustable 5100's give you more options and its a decent quality shock. You will be able to adjust your shock to three different settings.
It was the first mod I did to my truck. Adding the 5100's all the way around. I set the front at 2.5 and added a 1" rear block for a lift. Then I upgraded to the TSB leaf pack for the rear and retained the 1" block for about a 2.5 rear lift. (Slight rake - nice stance). Be sure to get an alignment after they settle a bit to avoid camber due to the front lift. Most people don't and you will see uneven tire wear and a small amount of vibes after a few months with the billy's set at 2.5.
I just finished installing Camburg UCA 1.25 Uniball - to prepare for the Camburg Spindles (next week). After the spindle lift I will re-set the front 5100's to .85. I wanted a lift without maxing out the front 5100. The spindle lift with provide me with more travel, a higher lift than the 5100's and provide better better/more alignment options - and allow me to upgrade to better shocks to create a decent mid travel set up. BTW, Have you seen the Camburg spindles - they are bullet proof and beefy!
When I first bought the truck in March of 2010 I ran into a guy that did the spacer lift. I liked the look and thought that was cool. I called a local Off Road Shop (Full Tilt in San Clemente, Ca) and they talked me into the Billy's. Best move I made.
Looking at rear suspension upgrades now to work better with the spindle lift. I will remove rear block and change out the TSB leaf pack to something a little more better. I figure I could run this set up for a while....Until I save a little for upgraded shocks. (FOX, KINGS, Camburgs). Then the 5100's will be replaced.
I will dial in the suspension (cost estimate around $3000/$3500+labor) while I wear out my stock tires and replace them with new tires and rims ($1000/#1200 tires - ??? rims (undecided).
Yes - I have the bug and love my truck!!!