Definitely rapier. Proven to be best for training pretty much everywhere. And only place CLS is better against bosses is Bandos and Saradomin in GWD. And perhaps TD's if you're not great at switching fast.
I would get the rapier, just because it rapes slayer and it's fast, also good for pvp. I don't know why the staff is in there, SOL and Zuriel's are much better for pvp.