Want to Buy "Gears of War" or "Resistance Fall of Man" for PC or XBox

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What's the best price and where can I get "Gears of War" or "Resistance Fall of Man" for PC or XBox?

I don't think "Resistance Fall of Man" is made for PC.

Is Gears of War better on PC or XBox?
I'm leaning towards PC just so I can play it on more than one computer if that's allowed.

Thank you