I use Photoshop CS and Sai to color. I used to use only photoshop, but recently I've been using Sai a lot. There is a TON of things that can be done with photoshop. When it comes to filters and effects Photoshop is the best. Sai however I think is wonderful! <3 It beats photoshop for just siting around doodling. The pen smoothing on Sai makes it so much easier to ink things. I also love how simple it is to make a work that looks almost hand painted. >,> Price is great too. My licensed copy was like $51

Photoshop:
Pro: Lots of options, extremely powerful for art and web content.
Con: Price. I think even my student copy set me back $500 for the entire suit. Pen smoothing is not very good.

Sai:
Pros: Awesome drawing tool geared especially towards artists. Easy to use and is very intuitive. It carries a lot of features similar to Photoshop and painter. IT HAS LAYER MASKS! <3 I use that a lot in photoshop. CHEAP. Thats important. Not many people have $500 sitting around to blow.
Cons: None really. Less layer effects, but you don't really need them.