How to decide between Gmail and iCloud for additional storage?

I’m getting messages both from my Gmail account and my Apple iCloud. It says I can’t back up my phone because I’m out of Storage. Gmail says I’m not going to be able to send a received emails soon because I’m out of space. Where should I add more space? Also have two photo apps, Google photos and the one for Apple. Should I delete one of those apps or what should I do? I’m not savvy when it comes to all this.

[Re-Titled by Moderator]