By nature and by design an inline six is about the smoothest engine built. General rule of thumb is the more cylinders the smoother, so the five will be smoother than a four, but not as smooth as a six. A straight eight is even nicer. A V configuration creates some horizontal forces that are not present in an inline, so there is often more vibration. The exhaust of a V sounds better, though. At it's most basic think Harley Davidson. Much is written about that vibration, sometimes in the letters to Penthouse.