I used to be hardcore "Mushrooms are better than fruitbats!" but recently I've changed my mind.

It all depends on your playstyle really. Mushrooms are good for if you like to spend a lot of time in the mines and want easy access to life elixers, and they're good for some quick cash early on in the game. They can also be used for the community center.

The fruit bats, while not as consistent, are useful as well. Fruit can be used for the community center without having to plant a bunch of fruit trees, which are expensive early-game and take twenty-eight days to grow. Bad timing could mean that you have to wait an extra year to get the fruit you need. They are also good for some extra cash, especially if you turn them into jam.

I normally don't bother with making life elixers, so I've found the fruitbats to be much more useful to me.