The PM25 comes with a factory belt drive installed instead of the notorious plastic gears. Also, the grizzly mill column bolts to the base from the back side whereas the PM25 bolts down to the top of the base. Arguably, the PM25 attachment system is easier to tweak and once trued up, won't get bumped out of alignment. The Grizzly would be more prone to shift along the x-axis.
Also, I believe that the grizzly doesn't come with the 3-bolt head mod standard like the PM25 does. This makes for a much sturdier head attachment to the carriage assembly.
Other than that, you probably wouldn't notice much difference.