Here's the deal:
The tension shackle works great with lower lift springs and especially with higher end springs like our custom stuff. The reason why is that when the spring goes to full droop the shackle pulls forward and lays down roughly horizontal so the tail of the spring is dropping 4-6" depending on the actual shackle length. This is 2-3" of "free" droop travel. With a compression shackle at full compression the rear spring elevation is basically the same as it is at full droop so you don't get that extra droop travel. But there are some buts...
With our custom springs, we can only get to about 4" of lift in a 52" rear spring with the tension shackle before the arch gets high enough that it's hard to even get the spring to bolt in. We can hit 6" of lift or a little more in a 64" spring. At this point we use a longer shackle which gives you more of the droop effect and requires even more arch in the spring so it's a balancing act.
In the regular "lift kit" spring world, we've seen good results with springs up to 4" of lift with the tension shackle but when the arch gets deeper the springs are generally so much stiffer that you're better off with a shorter arch spring and a shackle flip.
This all depends on having urethane bushings in the spring eyes so that everything can rotate freely. As soon as you have a rubber bushing or a binding urethane bushing this all goes out the window.
In a lower lift world the shackle flip shines for economy and reasonable performance. Given lifted spring prices, the $200-ish for a shackle flip is attractive if you have a decent stock spring to start with. Once you get over about 4" of lift the shackle flip/lifted spring combo rules since spring arches stay reasonable and realistically you can't even get a rear spring over 8" of lift so if you want more than that without a block you have to run the flip.
The tension shackle has some desirable effects on spring rate as the suspension cycles that are possible to approximate with a compression shackle but it requires a bit more finesse. Basically it's hard to screw up a tension shackle system but easy to screw up a compression shackle system. In the real world as long as you stay away from extreme angles with a compression shackle this last point is not really that important.
I also think there are some handling details that go back and forth. A deeper arch spring is going to have more roll steer but a compression shackle system is usually set up around at least one factory mount and it can mess with handling through axle steer also. These are the really fine details that don't really make a difference to 99% of the world.
clear as mud?
