Oh, I forgot- there's also ArtRage Touch in Windows Store- I think it's paid, but doesn't cost too much. That said, desktop version of ArtRage is already great on Surface, because UI is very well tailored for using with touchscreen
I've tried it out and quite like the app but it is crashing out every now and then. I'm on the trial version but it's not clear what the differences are.. and I have millions of days on the trial!! Not sure whether there's a trial of the desktop version.
Just downloading the trial of Corel Painter too, though it's a LOT more expensive!