Office 365 Groups are a quite old concept. They were first introduced somewhere in 2014, but since then the concept developed, from a simple Shared Mailbox, into a tool dedicated for collaboration between employees. Then, in November 2016 Microsoft Teams went public what… brought a lot of confusion on users’ faces. I guess this confusion is still present.
I this post I am not trying to explain when to use what, but to compare those two products to show you their capabilities and… to help you answer on your own, which tool to use when.