Woops, totally read your question wrong. You already have Sai, d'oh. Disregard most of that, but Photoshop is still good!

Pirate them both. Sai can open Photoshop files!

Hm, what? Who said that?

Anyway, I use both and I find that there are certain advantages Sai has over Photoshop and vice versa. Sai is slightly more user friendly but offers significantly less functionality than Photoshop. Photoshop offers more tools and editing powers, but it can take slightly longer to produce what you want (if you're relatively unfamiliar with the kinks like I am).

Personally, I sketch and edit exclusively in Photoshop and do details/outlining in Sai. The picture to the left is something I did in which I used both programs (just view here: thehalfanese.deviantart.com/art/VENGEANCE-308647478). On the character/motorcycle, all of the outlining, the white shine, and a bit of the coloring were done in Sai. I find the default Sai brush is capable of producing very smooth and sleek lines, so I don't waste time trying to program a custom brush in Photoshop. On the other hand, the sketching, blood patterns, and text were done in Photoshop and could have only been done in Photoshop. Sai doesn't have a text tool, just so ya know, and I have no idea if it can do custom brushes (though I'd assume it can). It's just easier to produce and manipulate abstracts designs like these in Photoshop.

Ultimately, I think you'd be getting more bang for your buck if you got Photoshop CS5/6 whatever. Sai is great, but I just think you'd be giving yourself more with Photoshop, even if it isn't exactly the greatest in every field. Hope this helps.

Thanks for the input, and on my old computer I pirated photoshop but never ended up getting around to use it, on my new pc I tried pirating it and my computer didn't accept it and i was forced to delete it. And I would really hate to pay 200 bucks for photoshop when art is just a hobby and I draw on paper too.

I bought paint tool sai since it was significantly cheaper then photoshop or Illustrator but a majority of the art I see is done on photoshop and the only thing adobe explains about photoshop is its editing rather than coloring and drawing.

Also thats a really awesome drawing

Thank you. Don't know if you saw, but I realized that I read your question wrong and saw that you already had Sai. Hopefully the advice on Photoshop is still good, though.

And yeah, Adobe doesn't really talk about PS's drawing capabilities. I've heard people dismiss Photoshop for drawing solely because of this; Adobe didn't make Photoshop for drawing so you won't get the best results you can. I still think it manages extremely well, but (in my case) you do have to do a lot of fiddling to get things looking right.

I don't know much about it, unfortunately, but them "vector graphics" sound freaking lovely and I've also had one very Photoshop-saavy (editing-wise) associate recommend I use Illustrator to draw instead. Sounds like there's potential there to me.