SAI doesn't have that big a variety of brush shapes, and creating your own doesn't give the most... desirable results (it doesn't quite work like the brushes do in Photoshop). However, SAI has more versatile tools than Photoshop. In my opinion, regardless of what type of art you're going to do, SAI will have you covered. I don't do very anime-oriented work anymore, but I still rely on SAI very much to get the job done. I would have a program that's at least like Photoshop (I use Pixelmator/GIMP) nearby in case you need it, since SAI lacks a few fundamentals that the other program will have.
I use paint tool sai for anime and non anime....but I wish I had started out using photoshop. In photoshop there is a wide variety of brushes and tools, and while sai is simple and non threatening, it just doesnt have as much.
I recently looked up Photoshop CS6 to see its price, and it seems that the only way to obtain ownership is to pay a monthly fee. I assume this means I have to have an account with Adobe? Is there anyway I can just pay one price in full for the program?
Any semirecent version of photoshop(CS3/4- CS6) would be suitable. You have to remember that SAI and Photoshop are both just tools. They both have learning curves. Photoshop has the capacity to do many other things, so if your intrest broaden or change you already have a superior raster image editor. Plus if you want to create photorealistic images it might help to have some of the tools in photoshop so you can change the lighting of the image easily without repainting. Photoshop can also handle lineart just as well as paint tool SAI. Just google a couple of digital paintings using photoshop and you will quickly discover that. In short I think photoshop with suit you the best.