It seems to me that people separate into camps: SAI, Photoshop, etc. I think each program has strengths and weaknesses but the most important part seems to be just getting familiar with a program. Personally I think SAI's best feature is sketching and inking, PS seems best with coloring and shading. But both programs can do both things, they just do it differently.

One thing you might want to look into is the pen stabilizer that works with PS. From what I hear it's kind of like SAI's pen stabilizer, which I find invaluable.