Not absolutely sure about the difference, but i think it's supposed to be "better" because you can make your own brushes and stuff like that. With Photoshop you have to install them. It also will expand your canvas for you in Sai, but those are the only differences I've noticed so far.
I used GIMP very long. I think it's good, yeah, but I couldn't work on larger images properly, not to talk about the lineart. The bigger brushes are so slow and it made my pc crash far too often. SAI has very fast brushes and is made especially for painting. I was sceptical too, but now I love it. :> And photoshop is too complicated for me. >.<
I do believe GIMP fudges up with larger PNG files. Once I tried linearting these pictures and it froze on me. I bet SAI would be better than GIMP. My friend helped me download and I will play around with it more before Christmas time.