I like alternating between Photoshop and PaintSAI cuz there's always something the other program lacks in. I like painting backgrounds and backdrops in Photoshop while painting people/animes in PaintSAI.
I guess you could say they complete each other
I used to paint with a mouse but I completely ditched it for my new tablet
Paint Tool SAI all the way. It is the only one that I have found that has an easy user interface and works smoothly. Wonderful for Manga, but I have yet to try realism on it. I'm not sure it would work well with realism for the lack of realism tools. But if you tend to draw Manga, I would HIGHLY recommend it.
I draw with a Wacom Intuos 4. I love it very much. Just make sure to check for new drivers every two weeks or so, to avoid lag of the mouse ^^