An adjustable unit will allow you to tinker with the ride if you like to vary it. If your stepping up t-bars or leaf rates considerably, most fixed rate shocks won't be able to control the motion, unless you get into more specialized racing type shocks, so adjustable are a much better choice in those applications.
So, what rates are you using, and how do you intended to utilize them on the car? Cruise to shows, spirited driving, autocross, road racing, and most importantly, how much are you willing to spend?
