Whips are already great Slash weapons, but there really isn't a good Stab weapon. (The exceptions are 2h Zammy Spear and the new sword, which is slightly worse than a whip) However, many monsters are weak to stab--Metal Dragons, Strykewyrms, etc--and the Rapier rapes them. Rapier with 85 Dung also means you lay utter waste to the Frost Dragons, so there you go.
I have cls, defiantly better for bossing, I'd rather the rapier for slayer though, and pking, maul is fun, rapier is good there also. Really just depends what you're into doing and what you want out of it =]
id personally say cls, most people only use rapier for slayer. and since u want PvM like bandos and stuff id say cls since its fast and godsword strength.