I have both RE 5, 6 and Borderlands GOTY. I say they are both equal in fun for Co-op. Borderlands has mindless fun while RE 5 and 6 require you to pay attention more. RE 6 even on normal mode is making me have to come up with strategies and plans with my friend on Leon's campaign.
"Can you draw two M&Ms fighting with katanas?" -Me http://img16.imageshack.us/img16/9606/mmfight.png -aDubiousNotion
Borderlands GOTY if you are doing coop. That game has some of the best DLC ever made imo, and it comes with a voucher for it or a disc with all dlc. For a first playthrough as coop, with no previous experience, and using all of the dlc, will net you around 100 hours. The game has tons of replay value.
The one redeeming hope for Batarians is that they actually get their money's worth when they go to a buy one get one free sale at Lenscrafters.--PinkFloydman73