I won't go on a large testimonial rant about why Manga Studio / Clip Studio paint is so great, just (if you can) go check it out for yourself. I believe it's a game changer and could stomp the daylights out of Painter and for a lot of folks photoshop. Clip Studio Paint (which is the same thing as Manga Studio 5) has more features at the moment. Manga studio 5 from what I understand will be updated to Manga Studio 5 EX coming up this summer and will match all the features of Clip Studio Paint, but still even if it's missing just a few features it's still not bad. The biggest missing feature in Manga Studio that I like in Clip is the option to make your UI darker (photoshop CS6 style). It's like Sai, Painter, and Photoshop rolled into one very similar to Illustudio.
If you live in Japan (or have a Japanese IP) you can check out Clip Studio Paint and have it translated using the tools for translation Here
Or if you want a fully translated english version go for Manga Studio 5 Here
The shown interfaces on those sites look cluttered, but the program is fully customizable (i.e. photoshop tab style) and can be configured to users taste.