The Pirelli's have a different, longer- wearing compound on the GT version. Having used both the ST and GT, I found that the GT did last longer but was not as 'grippy'. I should have specified that in my original post and will go back and edit it.
From an article in Cycle World: "Available in 16- and 17-inch sizes, the Angel GT has one spiral steel belt in common with the outgoing Angel ST, but beyond that it’s totally new. As Pirelli explains, most wear occurs as the working surface slows under compression and then accelerates while rotating away from the road. Since physics can’t be changed, the company revised the tire construction to widen and shorten the contact patch for improved wear characteristics.
Grip is a function of the tire carcass conforming to the road surface, and the rubber compound’s ability to cling microscopically to it. Pirelli reports much progress in the formulation of the polymers and silica used to provide cold grip, wet-weather grip and longevity. An improved profile providing a linear transition from straight up to full lean, as well as a grippier tread compound at the sides, further expand the Angel GT’s cornering appetite. "
I am not certain that the same thing applies to Michelins but I believe it does although again, I am not sure. I have not used Michelin GT's and honestly, do not plan on trying them. I believe my next tires will be Michelin PR4's with a -55 series on the rear.
Brian
I thought the sidewalls were stiffer, not that it was a harder compound.