Although I know Photoshop for much longer than SAI, I can't really draw in it. My lines get way too wobbly without any stabilizer. One thing more makes SAI more attractive - illumination algorithm. SAI has the more realistic one, in my opinion at least. However, not even a percent of brushes PS has can be found in SAI. Being honest, stabilizer is the major argument for me...
yeah... the stabilizer. For me, the major pain of using PS is its blending - its brushes just don't pick up colors like SAI's, and it is frustrating. As for the stabilizer... it is ok while I draw lineless. Oh, and my way of coloring hair is totally not suited for PS.
I had the same feeling when I switched from GIMP to Photoshop. It was something like "WTH is going on here?! How to rotate things?! What's with the...", but one can get used to it after some time. And I will never ever ever touch Inkscape too, because I did it once and it was a horrifying experience (creating objects with figures? The less points the better? Where is my eraser? I can't fill that closed object? No thanks, it's not a natural way of drawing).
BUT you can combine those two programs to create interesting effects. I'm sure both of them have something that the other one doesn't.