I use SAI to make comics and switch to Photoshop for texts. And as everyone else, I just use the cracked version that is passed around by everybody. Even if it doesn't have all the brushes you can just add them later or make your own, and the lack of transparent backround is not an issue if you use it together with Photoshop (backgrounds will be transparent in Photoshop!). And I have Windows 10, and I've never had any issues.
However, CSP has much more options. And you're already used to it. I would advice you to get the cracked version of SAI from someone or just use the trial one until CSP goes on sale. It's REALLY cheap when it's on sale! I've seen it for around 15 USD.