I would go for the Sub first. While the Arc is a great speaker, because of the size of the drivers (this is true for almost every soundbar), adding a sub really makes it shine, because it takes the load of trying to produce low frequencies off of the soundbar.

Be careful though, if you watch a lot of content with Dolby Atmos, you will be itching to get surrounds as well.

Hi Pools-3015! Thanks for your recommendation! I don’t understand the last part “be careful…to get surround as well”, if you can explain it to me, I would appreciate it!! :)

It's just a little humor. The Sonos ecosystem has been a rabbit hole for me. I can't stop adding to it! Just yesterday, I bought a pair of Era 100s to replace my Play 1 as surrounds. Did I need new speakers, probably not!

Ahhh jajajajajaj, I understand, yes, probably it’s gonna happen to me the same! Have a good weekend! And thanks!

Depending on your likes (music and/or movies), and your home layout, I’m putting in a vote for the Eras. You can then try them as a stereo pair for pure music listening, and you can bond them to your Arc as surrounds to get Atmos audio. 

In case you haven’t noticed there are major issues with the Sonos app/system. 
 

Buy from a retailer who has easy returns in case you have issues with it.

Can you post your negativity somewhere else? Obviously, like myself, OP doesn't have any issues with the products they have. So why try and discourage them from enjoying their products even more?

No. Suggesting they ensure easy returns is being realistic. 

I did not discourage them from buying or was negative at all. You are being dishonest in characterizing what I wrote as negative. 
 

Don't underestimate the Sub Mini.

I use one with a Beam and a pair of Era 100s and the bass is on a par with my Sub used with a Playbar and a pair of Play:1s.

At the risk of being called “negative” I echo the earlier comment about the App and being prepared for some extra work in getting things setup. I was seriously considering replacing the Play:1s mentioned above with Era 100s before the new App “lost” my Sub Mini and I lost 3-4 hours of my life getting it back.

Yes, we had lot of problems when we were installing the arc, but fortunately after that it works fantastic!!

Basically I want something for both, films and music, what is best?? Thanks

I’d really lean to the Sub as the first upgrade.

I have two HT Rooms, both with a Sub Gen 1 and One or Play 1 surrounds and fool with turning either or both off to see how it sounds. For anything but 5.1 sound with a lot of surround audio the Sub makes a far larger difference.