No real preference, but this does remind me that MonadZip probably should have the following extra law:
uncurry mzip . munzip = id
This law is passed by all current instances and fits the intent of much harder to state opposite facing information preservation law.
Since we continue to insist on this class containing the annoying munzip operation, this law is actually far easier to demonstrate than the existing law.
We can also restate the other information preservation law now that Functor is a superclass of Monad to the rather more succinct
() <$ ma = () <$ mb ==> munzip (mzip ma mb) = (ma, mb)