[quote name='Frog Ribs']So if I wanted to pick up COD4 and M:UA, what would be the better choice for the third game: COD2 or COD3?[/quote]
I'd recommend that you get COD3 from this deal and then just pick up COD2 from GS/EB or even new from a B&M for pretty darn cheap. COD2 has been on sale for $9.99, $12.99, etc. COD3 is usually $14.99 or $19.99.
COD2 is made by Infinity Ward, the same team as COD4. COD3 is made by Treyarch.
Campaign - COD2 is like COD4 in that you keep switching characters and the story builds from multiple angles. COD3 focuses on Normandy and the battle that takes places there.
Multiplayer - Pretty much a toss up. But COD3 is better supported. The Gold Edition of the game comes with extra maps.
I like COD2 more than COD3, but they're both worth getting.