They are both very viable options but you have to take into account the resolution and the monitor size as well as the cost of both. personally i would go with the m15x (as long as u use vista 64 bit)
Personally, I wouldn't buy an Alienware. But since you didn't ask that, it would depend on your usage. Games that are only optimized for single and dual core CPUs will like the Core2 Extreme better because of its higher clock speed. If your games are optimized for Quads, then that will be favored.
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