Sai is for drawing, there aren't that many bells and whistles compared to photoshop so you mostly just focus on the drawing aspect. It has a lot of different brushes that try to mimic a certain kind of blending and to be honest it's really great for illustration. Especially if you rely on line work since it comes with a "stabilizer" a small portion of the program devoted to making sure your lines come out as smooth as they can by adjusting settings. Honestly if it came with a text tool I would still be using it just because I don't always need a lot.
Photoshop is more about manipulating things and using special effects to improve the quality of images. It's not to say you can't draw with it, I've seen some amazing stuff with photoshop, but it's focused on having a lot of different features for you to use. Which can be great for comics and also bad since it means a learning curve and things can get complicated. But because it has so many options you can do a lot more with it, especially since you can customize plugins and brushes.
I'll admit once I got over needing a stabilizer I was content with photoshop and was more then willing to learn lots of more tricks to have fun with the program.
Both programs are greta and have their pros and cons so it can really depend on the artist.