How did you even come up with this combination? They're not similar! CoD is a generic online shooter. Half Life 2 is a much older game, that is a single player shooter experience with some vehicle combat that has a great story. Half Life 2 > Modern Warfare 3 in several different factors(gameplay, story, etc.) , but MW3 does have multiplayer, and better graphics (which doesn't even really matter).
Also, one is from 04 and one is from 11, seven years of technological advance is not fair for any game...
Regardless, I've played both and Half-Life 2 blows the McVideo Game MW3 out of the water in terms of innovation (of the time), storytelling, overall fun in the gunplay, and the overall impact the game had (one sold a lot, one is considered the best shooter of all time, I'll let you guess which is which)
