I am looking to lift my 2006 GMC Sierra 3500 4x4 Dually. I do not know which lift I should go with, either the 6" or the 8" by CST. The 8" will look alot better, but what would the difference be between the two be. I tow a toyhauler that weighs about 15,000lbs. Does any one have suggestions either way 6 or 8 and why. Would there really be a difference between 6" or 8" when I tow.

I wouldnt tow 15000 lbs with a lift its probably really bad for your drivetrain.

The CST 6-8" is the same kit Just cranked torsions And different blocks.
Just start with the six and if you want to go higher when the tires wear out you can. At 8" the ride will be rougher. I have thier 9-11" kit and my toy hauler weighs about 12,000lbs. tows fine.