SAI and PS both have their pros and cons. Personally I'm only using SAI because I only draw, and for that it is in my opinion the best because it's simple (compared to PS) but still offers a lot of functionality. Sure you don't have that many fancy pens and stuff (unless you make them yourself) but I always found PS to be just overloaded with functionality you often don't need just for drawing. Of course for some fancy effects PS is better suited since SAI offers not much in that regard. Another pro for SAI is of course the price (if you actually buy it), but probably the biggest con for SAI is that it's discontinued, i.e. noone's working on it anymore :/
There is a blender tool on SAI that you are able to use efficiently. While Photoshop does have a blender-like tool, is is harder to use and takes more time to learn how to use it effectively.
I would suggest downloading it and maybe watching some good tutorials on how to use SAI correctly. It's a good program - but it is nice to have both SAI and Photoshop, as using them together can add effects that look marvelous! <3