The main differences come down to adjustablilty and the assembly of the coilovers
The BK0510 kit for $629 uses the 5100's and the ToyTec
/Eibach coils along with the rear AAL's and a diff drop
The TTHA-CK kit for $899 comes assembled with the ToyTec aluminium top mounts, axle shims, and U bolts.
The Ultimate kit is similar to the TTHA-CK but the front coilovers are a threaded body style coilover so it give you the abilty to fine tune you lift height and adjust for extra weight
Let me know if you have any other questions about any of our kits