...sure might be appreciated.

Hi. So here's the thing. I can't decide which to use between Photoshop, Corel Painter and SAI. I am leaning towards oils aquarelle effect painting rather than photo-realistic painting. So... suggestions?
I searched, and understand that SAI is great for line art, anime and comic coloring, but can it do even better? Gouache and aquarelle feels? I know Painter can, but can photoshop achieve these effects too?
Putting layers and brushes away here. I am talking more about how the colors react when painting.

Also, can we greyscale in Painter like many do in Photoshop, so as to get to work with values first?