I think that Allegorithmic are doing a great job on Painter. We are looking at version 1.0 here and I think it's a very solid release. Pretty sure they are aware of the "handpainting" needs of us game artists. They are listening to responses and thoughts so let them know what you want/need.
i haven't tried handpainted texturing with mudbox, but mudbox's texturing tools and interface are awesomely simple. and for 10$ a month that's super cheap and you get other awesome things with it. with mudbox you can also export layers directly to photoshop, which is very nice. not sure about SP because i haven't tried it…