I'd say go for FFXIII first. By the time you finish it, NKN might be available for cheaper. FFXIII is not as bad as many made it out to be. I enjoyed it, except for a couple characters that made you scratch your head.
The game is only about 25-30 hours long minus the post game quest, so it wont even last him.
Its commonly known on Gamefaqs that if anything is more evident than proven facts, its personal opinions.