Whats' the different brew and gem install?

Recently while running pod repo update, it ask me

CocoaPods 1.15.0 is available.To update use: `gem install cocoapods`

So I run sudo gem install cocoapods. It shows all good

Successfully installed cocoapods-1.15.2Parsing documentation for cocoapods-1.15.2Done installing documentation for cocoapods after 1 seconds1 gem installed

To my surprise, when running pod --version, it sill shows 1.12.1

After thinking a while, then I realize I have to run brew upgrade cocoapods.

My question is, when would I know I should run brew vs gem install?Are they stored differently? (Do I have 2 cocoapod now?)What are their differences?