Cobalion is the better of the two. This is due almost exclusively to its better typing. The Steel type is a much better type defensively than the Grass type. It is also more offensively relevant on a Fighting type Pokemon, providing Cobalion with a hard hitting STAB move that is super effective against Fairy types. Steel/Fighting also just has better coverage than Grass/Fighting. Cobalion also has access to Stealth Rock and Volt Switch, two wonderful moves that are always handy to have.