Magic Bullet is very nice (I use it myself), but was primarily designed for de-interlacing your footage to give you a true progressive-scan source to work from. And it does that very well.
That being said, it does have a very nice set of collor correction filters, all of which are very adjustable and animatable over time. On my last project, I used Avid Xpress DV for all the editing, and color correction; but after it was finished I got Magic Bullet, and after re-color-correcting a few chunks of the movie, I was much more pleased with the Magic Bullet results than those I got from Avid. Keep in mind that I'm not a complete expert in using either one, but Magic Bullet can do very nice things for your footage. It also costs $1000 and renders somewhat slowly. And in Magic Bullet, you can't really adjust an individual color very well. There are no color wheels. But Magic Bullet does have Black and White Diffusion filters, which are quite nice. I'm not sure if DigiGrade will have those.
In Digigrade, I do know that adjustments will be fully animatable over time, but really not much is known about the specific features at this point. I am reasonably sure that you will be able to adjust levels and lightness, as well as saturation, brightness, hue, etc., with independant control over highlights, midtones, and shadows. I'm not sure about curves. But really, I 'll have to wait and see what it has and doesn't, just like everybody else.
Keep in mind, too, that the tools and features available in DigiGrade will be constantly being updated. (is that proper English?)
Magic Bullet is very nice, but is really designed to create a 'Film Look' from video. From what I understand at this point, DigiGrade will enable you to get a better 'Film Look', but that isn't what it's basic purpose is. It is designed specifically for color correction and grading, and will have an impressive set of tools for that job. It's cheaper, too.