In 32 bit (the only way to play IMO) the Radeon is much faster, especially at high resolution. I would list some other reasons it's a better card but then people would probably start arguing. And I agree with Mustang. There is no point trying to argue with people if they haven't got it by now. But then, I really don't like to argue anyway.
And to the person who said that 32 bit color is not noticeable to him, I can only say that most would disagree. Then again, since you have not seen 32 bit on the Radeon, it's probably whatever card you are using. Afterall, the Radeon's 32 bit image quality is absolutely beautiful, and most reviewers have commented on that.
Anyway, 32 bit does make a difference to most, and especially if you have a good monitor. Things look much richer, and much deeper in 32bit color. I believe Anand has even said that 32 bit is the "preferred" way to play.
So if you only have a small monitor, only care about 16 bit color, and the monitor can't handle the high resolutions, then the MX may be your best choice, so look around and find one for as cheap as possible.
But if you want to play at high resolution, 32 bit color, have the monitor to handle it, and want better overall performance and features, the Radeon is the best choice.