Aside from the appearance, what's the difference between the Pro and Stabilyx tights?

They target slightly different muscle groups for additional support. The Stabilyx tights provide support for the lower abs, hips, & back, along with knees & calves. The Pro tights on the other hand provide less support for the core (focusing primarily on the lower back/hip, with no compression over the abs), but do have much more support for the upper leg (quadriceps/hamstrings), butt, & shin/calf. Overall, both tights are made of the same material, but the additional compression panels are located in slightly different locations.