Bioshock Infinite is faster.
...
I was expecting it to last longer than that.
...what. They're both about the same. And for being recent AAA titles, they both last a surprising amount of time.
Bioshock Infinite takes 12 hours, on average, to finish the main story. 26~ hours for everything the base game offers. Tack on another 3-6 hours for each of the Burial at Sea DLC, and 7 hours for the Clash in the Clouds DLC.
Far Cry 3 takes 16 hours for the main story (not a huge difference), and 37 hours completionist. +9 hours for Blood Dragon.
tl;dr they both offer a good 50 hours with DLC, 30-40 without, depending on how you play. The only difference is how linear Bioshock is.. owait, FC3 was, too. =p
Regardless.
I voted for Bioshock Infinite. As much as I loved the first, Infinite was probably my favorite in the series... if not a very close tie. If you suffered through the crap that was B2, you should definitely give Infinite a go next, and round out the series and the story. It's well worth it; more-so if you cared about the story at all. Burial at Sea kind of ties everything together.