Should I buy Red Dead Redemption or Far Cry 2 and which one has better replay value

Red Dead Redemption is already considered one of the best games released this year and many downloadable packs will keep it's replay value up there. The first first pack Legends and killers is said to be released later this month with a release of the Undead Nightmare pack later this fall. Two other packs will also be scheduled for release sometime later. Far Cry 2 is an older game that costs half to one third the price of Red Dead Redemption. The value is unfair to decide because Red Dead Redemption also has a larger online audience and is so popular that new downloadable packs will also add to the price. Links to both games Wikipedia site has been added under the related links below the answer information.
Far Cry 2 can be found on Ebay in used condition for less than $10-15 for the PC, Xbox 360, and PS3 and at Gamestop in preowned condition for $20