I think the GP Extra has come a long way over the previous big ARFs that they have made to the point where it is a very good flyer. They have learned some lessons in the airfoil design that make it a more stable flyer at slower speeds.

I think it is a more solidly built plane for a gasser over the DP Extra, but is also heavier and the wing loading is higher as mrc100 mentioned.

I also think that although a great flyer, the GP Extra is not the 3D plane that the DP Extra is. In the right hands, just about any plane can hover and torque roll, but the DP Large planes redefine 3D for the average RC pilot. They fly bigger and perform better in this arena.

IMHO, if you want a plane to practice basic IMAC, or kick around with some great sport aerobatics the GP Extra is a nice plane to do it with. I think the DP Extra take you to another level without much effort.

As far as the covering, there's a million stories about loose covering, missed glue joints, etc for just about every plane out there. My DP Extra is flawless, but then again, so was my Modeltech Magic and I don't necessarly put them in the same league as DP and GP. I think you have to balance out the overall comments about the finish of the planes and think you will find that most owners of both really love their planes for what they give them.