MonadPlus instance for ContT

[Ross Paterson]

Alistair Lynn has proposed the following instance:

  instance (Monoid r, Monad m) => MonadPlus (ContT r m) where
    mzero = ContT $ const $ return mempty
    m `mplus` n = ContT $ \ c -> liftM2 mappend (runContT m c) (runContT n c)

but this would also be possible:

  instance (MonadPlus m) => MonadPlus (ContT r m) where
    mzero = ContT $ const mzero
    m `mplus` n = ContT $ \ c -> runContT m c `mplus` runContT n c

Is one of them better?