Tablet for Painter. Mouse for Pixel.
Tablet pens are meant to facilitate natural drawing. People skilled at drawing obtain reflexes and formulaic control over works with a pen, and while the control is there, the ability to use a Pen for fine pixel details and pushing does not feel natural, at least in my opinion.
If you learn to draw with a mouse, you can actually do quite a lot, even in the cg illustration and photo manipulation realm, though a tablet pen will take up where the mouse is lacking. Ultimately, though, the design of a mouse is much more suited to doing pixel art, so don't buy a tablet just for pixel work. Also, it's a real pain to snap straight lines and similar mouse friendly actions with a pen.
If you're interested in pixeling your drawn designs, then yeah, definitely get a tablet, then create the design, shrink it, and create a pixel version. I mean, if that's your primary intent, it's better than purchasing a scanner.
Scanners are great too, but more for the graphic designer and typographer than the illustrator. With the advent of digital cameras and digitizers, they're not as valuable, but still very handy.
If money is no option and your visual art interests are boundless, get both.
