[DickTees.net]

If it's in your budget you'll want to go with the 9"x12".
I have a tablet at work an almost never use it when I am working in Illustrator. The strengths, for me anyway, of a tablet lay in what you accomplish in Photoshop with it.

 

[DickTees.net]

So why is a mouse better than a graphics tablet for Illustrator?

Personally... I use the bezier pen tool to draw/model images in Illustrator. It's just easier for me to lay down points with the mouse. That's just the way I learned a decade ago. I do get a lot of use out of my tablet in Photoshop though.