I have been using Paul Gage tires almost exclusively now for several years since I acquired my painted mdf track. At first I bought just the pgt tires but later after trying the xpg tires I began buying only xpg tires. About a year after switching to xpg I started noticing the tires changing color to an orange brown hue and eventually loosing cohesion and falling off the rims, both plastic and aluminum. After checking many of my cars I noticed that the cars I equipped with pgt tires were still OK while most of the cars with xpg tires were either starting to change colors or had already fallen apart. So my take on the situation is thus, on cars that I only drive occasionally I will replace when needed with pgt tires, not as grippy but not subject to rapid deterioration. On cars that I race often and feel the need for the extra grip I will continue to use xpg tires as they usually wear out before falling apart. This not very different from the situation with 1/24 foam rubber tires, if you race often they wear out after at most 3 or 4 races, but if you don't race 1/24 very often you usually have to replace dried out tires before going back to racing.
So for cars that sit on the shelf most of the time I would buy pgt urethanes and for cars that are raced regularly I go with xpg urethanes.