The Sunny variant of Cherrim is significantly better than its Overcast Form, primarily thanks to the addition of Weather Ball (FIre) from the April 2020 rebalance.

What is the difference between Cherrim forms?

Cherubi evolves into Cherrim, and Cherrim can have two forms: Sunshine Cherrim or Overcast Cherrim. In the mainline Pokémon games, Cherrim's form would change on the fly - if a battle had Sunny weather conditions, for instance, then a Cherrim would appear in battle in its Sunshine form.

Why are there two types of Cherrim?

Cherrim was originally planned to learn Power Whip, but it was removed before Cherrim was released. Initially, only Overcast form could be obtained from evolution. It was changed to be randomly chosen between two forms.