I would go with the pair of Era 300s because they give you the option to listen to music in Dolby Atmos which sounds phenomenal on the speakers. And a pair of Era 300s produce a lot of bass even without a separate subwoofer.

Thank you.  Complete ignorance here, but is Dolby Atmos for music or only home cinema? What music sources would I need to play to get this enhancement? 

I would agree given you can then listen to Atmos via Apple or Amazon. I have 2 Era 300s for music and love them. I do have a sub mini with them mind you, but they are fine without

In your situation, if you get a pair of Era 300s, you will only be able to play music in Dolby Atmos. You can play music in Dolby Atmos directly from the Sonos app with an Apple Music or Amazon Music Unlimited subscription.

As @GuitarSuperstar states, if you subscribe to Apple or Amazon unlimited and stream through the Sonos app, you will get some tracks which are Atmos. I tend to look on my Amazon app for Atmos tracks first then stream them through the Sonos app 

Mind you, normal stereo also sounds fab! 

Thanks both. Very helpful. I prefer two to 2.1 on principle, so this confirms my thoughts. 

@SarahN  I could add a submini later… is it worth putting this into my Sonos planning? Does it make a difference?

I feel it does add a bit more low end, but the 300s are excellent without. Like you say, you can add one later on