Limbo takes thinking outside of the box, and has fun and unique puzzles that you wouldn't expect to be in a video game. Shank, however, looks like a hack/slash shooter that all you do is go around and kill people, requiring almost no critical thinking.
Limbo would definitely hold my attention longer, and be fun on the second playthrough, whereas Shank looks fun at first, but it gets dull fast.
IMHO, I'd say go with Limbo.