Hi there! Keeping the main brand as the domain has a few advantages:
- Short domain names are easier to remember, easier to share, and have a smaller chance of resulting in typos. (ex. 20 characters vs. 9 characters)
- It's also just a good idea to have content from one brand in one place, and not spread out of many domains. Not exactly applicable post, but you get the general idea.
- There are ways to move domains/brands. But there is a chance of loosing authority, brand awareness and rankings. Here's a post on how to help with the transition if you find it necessary.
I hope these resources help! 