For “pure” stereo, Sonos’ literature says stick with the Fives. If you want to listen to recordings in Atmos, go with the Era 300’s. 
To muddy the waters though: whilst I don’t listen to much in Atmos, the Era 300 pair I have are pretty good for stereo music sources too. 
Do you have anywhere you can audition them?

I suggest going with Five’s vs Era 300’s. I have both and IMO the Fives provide a slightly better sound stage left to right. The Five’s can be positioned vertically or horizontally. Each positioning will give the Five’s a different presence to allow you to decide which orientation is better. The aforementioned is also discussed on the Sonos website reference the Five’s. The Era 300’s can only be mounted in one position. However as was suggested listen to both in stereo pair. I suggest doing so in your home and returning the speakers that aren’t to your liking.

Indeed listening to both is probably the best to decide. Ordering them both and returning the ones I don’t want to keep, or keeping all 4 of them .

I don’t even intend to say goodbye to my old speakers. I still find them very good. I would like to have all speakers I listen most to in the S2 app though. I also have a play:1 and an Era 100 that are already in the S2 app and having my living room speakers also in there would be handy. (I’m one of the few people not having any problem with the new app).

Thank you both for such quick reply !

The auditioning is almost as much fun as the on-going listening. Enjoy! 

 For classical music I would favor the Era 300s.  Classical orchestral is awesome in Atmos.  Add a sub later for bigger sound.

And I did just that. I eventually kept the Era 300s together with a sub mini. It sounds really really good. The 5s are going back. They were as good as the combination era300s+sub mini, but I kept the 300s because they are newer. Thanks all for the insights.

Thanks for the update, @pjotr12