If you’ll be playing Dolby Atmos music from Apple Music or Amazon Music, then you’re far more future-proofed with the Era300s as they are built for Spacial music, as well as all-round power. They are also newer products and so may well be supported for longer than the Fives.

FWIW..the Play 5 (Gen 1) was introduced in 2009 and is still supported using the S1 app. The Five was introduced in 2020. So do the math 

Sonos say the Five for the best stereo/“pure” music, but the Eras are pretty good for stereo and give the benefit of Spacial play too. Plus, they are designed to eliminate the “sweet spot” associated with a traditional stereo speaker placement.
 

Have you tried listening to them anywhere, to see if your ears have a preference? 

Thankyou for your replies and help.

i have Sonos gear in the rest on my house, including  an Arc plus sub for films. I am tending to lean towards the era 300 now from what I’ve read, and your help. 
cheers

One question that wasn’t asked since you provided additional info…

Will the Era 300 ‘s be used as surrounds or stereo pair? If as surrounds then they are the better choice. However, when used as surrounds they cannot be played independently of the Arc.

I love my single 2nd gen Play 5 that sits by my reading chair and for stereo listening I'd be happy with a pair of Fives.

Since a lot of my listening in my office space is Atmos the 300s were the way to go. Adding a Sub is probably overkill but it sounds amazing when I crank it up.

Do be aware that some Atmos tracks are horribly mixed and sound far worse than the stereo version but it seems the worst are being fixed by the artists.

No not used for surrounds. Just music. I am removing my pre amp / power amp separates system out of the room. Currently use cerwin vega speakers, which are awesome to say the least. I am a little concerned about the number of cds I have and the fact I won’t be able to easily and tidily link the cd player into the sonos system. 

Edited now the spam has been removed.

I understand you can buy a converter from RCA out of the back of a separates cd player into the era300?? Has anyone done this? Can it be co trolled by the app?? 

https://support.sonos.com/en-us/article/play-line-in-on-your-era-300

My cd players sat unused once I'd ripped my CD collection to a NAS and linked that to my Sonos. Why be fighting CDs when you can just queue up the electronic versions?

Limited to about 65000 tracks though.