I had 5 GTs on Pandora, wore the sides of the front down to the wear bars in the cupping. This was the only tire set where I managed to wear out the front before the rear! Less than a year old -- I was not impressed. 4s lasted much longer.
I replaced the front only with a 6 GT. Michelin states that the difference between the 5 and 6 is that they have applied the "MICHELIN® 2CT+ Technology" that was part of the 5 rear to the 6 front. Basically, it's touted to stiffen the sidewalls under cornering and increase longevity. Time will tell, but I passed Pandora on to my son shortly thereafter so I'm not sure how much time it will take.
For the brief period I rode it with the 6 GT up front, I found the dry handling to be excellent, equal to what I came to expect from the 5 GT and several 4 GTs before it. I never had it out on wet roads though.
As for differences between the rears, I haven't come across anything definitive from Michelin. The fronts looked the same and photos of the rears look the same, so differences, if any, must be internal.