Everyone already covered everything, but here's my two cents.
I HATE Sai. The interface sucks, making brushes is a pain in the neck, I can never get stabilization to work, ectf. Once you get used to it it's not bad, but compared to every other art program, it felt like Sai was trying to do too much.
If you can get it I recommend Clip Studio Paint because they have resources galore right at your finger tips and they ad updates regularly that one day you probably won't even need photoshop. Until then, if you don't have CSP, photoshop is the better option. Photoshop has better options than the other two when it comes to fine details and image handling. But the basic brushes suck in my opinion, so you'll have to search online for better ones.