Fixed length look like the top image, adjustables look like the bottom. The only real advantage is the adjustables allow you to adjust caster seperate and apart from the alignment bushing. That way the bushing can be used solely for adjusting camber.
Just pick up the phone, call Tim and say hey man, could you stick the adjustable lower arms on my rig. Unless you prefer the fixed length. Im not finished with mine yet even though all the parts are here, so i cant give any testimony about them one way or another. I think there was a $150 difference between the two.
If you get the adjustables you can pickup bushings at the following link if they ever fail or you wanna run a sperical type end. https://www.ballisticfabrication.com...istic-bushings