Small company (8 people) - SharePoint Online Teamsites, Groups or both?

I agree with Salvatore Biscari's recommendation if you plan on actually using Teams. Teams is great, but if people aren't willing/ready to use it, it will be a lost cause. So don't go to Teams until you sit everyone down and say "this is how Teams works, it can be great, but we have to AGREE HERE AND NOW that we're going to use it". Channels are only a thing in Teams. They don't affect anything else. (Okay, they create folders in the default SharePoint library, but it doesn't stop you from organizing your SharePoint content the way you want outside of that library.)

Regardless, of which type of SharePoint site you decide to use, since the company is so small and permissions aren't really an issue, I would definitely use separate libraries for each side of the business (marketing, finance, HR, as you mentioned). It will let you scale better and also keeps things better separated. (Also, if permissions become a thing later, it's easier to do permissions with a library than a folder in my opinion.) I put together an everyday guide to libraries http://icansharepoint.com/basics-sharepoint-document-libraries/ if you're interested.