Ps3 games, need help!?

Im getting one game later today. I have a ps3, currently no online, but soon to be. I dont know weither to get Cod4 or Mgs4. Im pretty good at FPS but have little experience with 3rd person. Obliivion is a choice because of its long and excellent campaign. But ive heard cod4 is the best fps on the ps3 currently (stupid haze..), but it has a short campaign? But ive also heard it has a awesome online. As for mgs4, i dont know much, infact i havent played either of these series. Mgs4 is said to be the best ps3 game out also, but its only offline?? I know there is MGO but i dont have any idea what it is. Please help quick!!