I think the action on the GP's is a lot nicer than on the SP's... even after doing some polishing and spring replacement. If the decision is a close call between the SP and the GP, I'd take the GP every time. I have both and like them both, but the GP is a superior gun hands down. More luxurious to shoot, handles recoil much better, easier to be accurate with (because of the superior action), and one more round capacity. There are times I take the SP over the GP, but only because I am limited in the size and weight I want to pack. The 3" fixed sight GP is bigger and heavier than the 3" SP, but not by a huge margin. If you can handle the 3" GP, it's a much nicer gun to own and to shoot. But it you can't handle the size of the GP, then the SP is an excellent revolver in its own right.
Note that the grips on the 3" GP are smaller than the grips on the standard GP... and the smaller GP grips make a huge difference in the overall size of the gun.