If you play a lot of audio in Dolby Atmos, get the Era 300s. They include side-firing and upward firing drivers that are designed specifically to enhance spatial audio/Dolby Atmos.

Thank you. I should have added that the system is used exclusively for watching movies.

Thats easy.  ERA 300.

I’d go with the era 300 to create a 7.2.4 system as opposed to a 5.2.2 which is what you already have

Yeah go over to the ERA 300 discussion topic, describes all this, will get some really great atmos effect finally.  I bought two 100s for the bedroom and two 300s for my arc and sub in living room.

I understand how adding 2 Era 300s will create a 5.2.4 system (Arc counts for 3 and 2 Atmos, Era 300s count for 2 and 2 Atmos, Subs count for 2)  Where do the 2 that make it 7.2.4 come from?

The concept of discrete channels is thrown out when you are talking Atmos.  Atmos is a positional codec,, meaning a sound is sent to a position in the room, not to any discrete channel.  So what speaker is playing at any point in time is dependent on your system's setup, not what channel it is assigned to.

The Arc’s and Era 300’s side-firing drivers work together to play the two side surround channels. The addition of these two channels makes it a 7. setup.

Thank you! Question answered.

Just so I’m clear. I can add the 2 era300 to my system which currently is the arc, bass and two ones?

You can only add two surround speakers to a Sonos home theater setup. If you want to add the Era 300s, you have to remove the two Ones.

Do you think replacing my ones with 300s will provide a better surround/ atmos experience?

Yes, because the 300s are larger than the Ones and have upward and side-drivers. The 300s should produce a much wider and taller sound stage than the Ones.

I have a question. The era300 I place it behind my back or I place it in front of me on each side of the television.

Surrounds should be placed behind and to either side of the main seating area.