Feraligatr gets Low Kick. That's the same thing as Grass Knot, opposite spectrum--it's as super effective against Empoleon as Grass Knot is against Feraligatr.
Feraligatr would win in an even (err, one on one) fight most times out of 10, due to its speed. That doesn't make it better, mind you, but eh. Empoleon's Steel typing is a tremendous deal, but eh.
I like the Gatr better. Empoleon is more likely to last though in battle.