Early in Stardew Valley Demetrius comes to your farm and offers to set up a research project in the cave located in the northern part of your farm. The two options presented are bats, which will occasionally leave fruit in the cave, or Mushrooms, which will occasionally spawn in the cave. Demetrius appears to make the offer once you have accumulated 25,000G. This choice is permanent, so you will want to pick the option that works best for you. Here are the benefits of each choice.

Bats

If you choose the Bats, fruit will occasionally spawn in the cave. This is an excellent choice if you want to complete the community bundles as early as possible, since you will need several different kinds of fruit for certain bundles. Otherwise, you will need to plant and grow several fruit trees before waiting for the right season for the tree to grow fruit. The fruit doesn't spawn every day, since there is only a chance, which increases if there is already some fruit in there. The fruit never despawns, so you don't have to check every day.

Fruit is useful in the late-game too, since fruit that grows on trees is universally liked, so it's a great steady supply of gifts for villagers. It can also be used for wine as well, which can be nice if you don't want to dedicate a large amount of space for fruit trees. If you want a steady supply of fruit without needing trees, choose the bats.