I played through all of Y with a Metronome-only team, great way to add some entertainment to a game that's otherwise somewhat of a cakewalk. Both have pros and cons. I would however recommend eviolite Chansey over Blissey. It's more of a wall/staller since it has almost zero physical attack, and there are precious few offensive moves it can randomly draw that it can pull off. Clefable can actually hurt other Pokémon, and can utilise far more offensive moves both physical and special. It's more vulnerable defensively, though.
This would depend on what you're after, and why you would actually want Metronome on a Pokémon at all. Great for novelty teams, not much else.