Hi @alfporto,
Because you have a long term ambition to expand your music system, I would simply go with the one Era-300 as a starting point, it has a wide stereo image when standalone, although clearly that’s not as wide as two entirely separate Era-100’s, but the Era-300 plays Atmos music too. I also think you could even get away without a sub-mini as the bass on the Era-300 is particularly good. So that would be my own personal choice, but then start saving because a stereo pair of Era 300’s do sound incredible IMHO.

I’m not sure anyone could really answer this for you.  I would rank the possible setups as follows, #1 being the best.  Bo

1 - Two Era 300s + sub  

2- Two Era 100s + sub 

3 - Two Era 300s

4 - Two Era 100s

5 - One Era 300

6 - One Era 100

How much your budget will allow or how you want to build up your system is personal choice.  I probably would go with Era 100s + mini sub to keep the budget low and would fill up that space well., but Era 300s + full sub is really really good.

I don’t use the latest Era products; I suspect they are somewhat compromised for music in also serving TV needs. 

For music play in small spaces, I would go with a Sonos One pair. And add a Sub later as budget is available, as long as it can be placed in the same plane as the speaker pair, somewhere between the two speakers.

If that is not practical, I would use a 5 pair. 

In all cases, I would use Trueplay tuning for best results.

Based on what reasoning would you draw this conclusion?  The Era 100 and 300 can only be used as rear surrounds in home theatre setup, same as Sonos One.

Why would you recommend Sonos One instead of Era 100?  There is $30 cost difference between them with improved hardware and features in the Era 100.

Thank you all very much for your answers! As @Ken_Griffiths says, I think the key is that long term ambition, so I think I will start with the Era 300 and add a second Era 300 in the future. I don't want to repeat the same mistake as with the Bose and have to rebuild the sound system in the future.

One advantage of the Sonos One is it does Google Assistant. The Era 100 is SVC and Alexa only. Both SVC and Alexa have limited language support.

I have combo 1 and absolutely love it!

I don't own any Era speakers, so you know what?  I'm not going to give an opinion on something I've never experienced, especially one loaded with rampant speculation and an incredibly bitter bias.