Here's how I see it, it takes "different" skill but not more. Those who say it does should do some gameplay and actually research it. Halo 3 I think it's 30-40% skill and the rest PURE luck with spawning and timing. Also your weapon over theirs.
COD I think is just camping and one shotting. COD is not really skill, but advantage. A 5 star general has way more perks than a recruit or a colonel so if it came down to it a general" with more exp" would easily win just from a couple little perks, not really skill.
However if a recruit did beat a general then I would put that into the skill category of COD.
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I would say it's about 10% luck, 20% skill, 15% concentrated power of will. This is an obvious troll thread, but I'll reply anyway. CoD and Halo are very different. It takes a lot longer to kill someone in Halo, (1v1 combat) so you need to keep a good constant aim the whole time. In CoD it is simply whoever sees the other person first gets the kill. tl;dr. Halo 3 requires more aim, CoD requires more map awareness.