eARC and ARC are compatible. You’d still be able to get Atmos through Dolby Digital Plus. It’s up to you to decide if the extra features of the Arc Ultra are worth the extra cost. 

Hard to define ‘optimal’. You should be fine with either device. 

This is what my room looks like. 9 foot ceilings and no wall behind the sofa. Pros and cons of  purchasing the ERA 100 speakers in addition to the Arc Ultra. Would that combination work well in this type of room?

 Save your bucks for the ERA 300s as surrounds.  Makes Dolby Atmos that much better.  If you don’t care about Atmos the 100s might be good enough.

The 100s would work, maybe on stands behind the couch.

300s are a bit more picky on position but would likely be better in that big room.

Hard to find a worse room than mine or more poorly positioned surrounds but they still sound better than nothing.

I have been reading that there are some significant problems with the Sonos smartphone app.  How do these problems affect the setup of the speakers and the quality of the output? I expect to use these speakers for streaming and playing blue ray movies. Not as much for just listening to music.

 I have a setup consisting of Arc - Sub Gen 3 - ERA 300s.  It’s connected to a 3 year old Sony Bravia via eARC.  HDMI cable is not the stock cable, rather it’s a 2.1 certified cable.  It has performed very well with the rare HDMI handshake issue solved by turning the TV off then back on.

 This setup was “bonded” and had TruePlay performed on it prior to the May 80.x app launch.  So not sure if all that would behave using the new 80.x app.  Others may have done those processes successfully on the 80.x app.  Maybe they will chime in.

 Dolby Atmos music sounds excellent on that system.  I’m guessing the Arc Ultra would make it sound even better.

Setup issues seem to be mostly fixed at this point. Sonos is working to get things completely fixed but it is a bit slow.

The controller has nothing to do with the music, it is just a fancy remote control.

Thanks for all the responses. Still a little confused  on what to buy, though.  
Picture of the room is above. 

Keep in mind my Sony TV has an HDMI ARC input, not an eARC input.  Does that matter?

If I purchase Arc Ultra and ERA 100 speakers for the surrounds, will I be able to experience Dolby Atmos? 

Do I need to purchase a special HDMI cable?

Thanks for your help. 

Getting stuffed again for living in the UK it seems. The Arc is $799 in the US but we have to pay around $1300. I’m so fed up subsidising cheaper prices in the US. There is absolutely no way Sonos can justify this price difference. 
 

Sonos (and companies like it) seem to take price differences between the U.S. and the U.K. to an infuriating level. The price jump isn’t just a minor tweak for import or tax differences; it’s practically a whole new tier of profit at the consumer’s expense. While some price variation is expected due to factors like VAT (20% in the U.K.) and import duties, that gap—$799 in the U.S. vs. £999 in the U.K.—is way beyond those adjustments.

A few reasons for this that companies sometimes throw out include “local market conditions” and “exchange rate risks.” But for tech companies, these explanations can feel pretty flimsy. Exchange rates fluctuate, yes, but that doesn’t come close to explaining a 60% markup. More often, it’s a practice of pricing based on perceived demand and income levels. Companies know that the U.K. market is willing to pay (or has fewer alternative choices), so they keep the price high to maximize profit.

Another element here is price anchoring, where they set higher prices in non-U.S. markets to position the U.S. price as an enticing baseline, creating the illusion of a better deal. This pricing approach could be considered a strategy for steering consumers in other regions to accept higher costs simply due to lack of alternatives.

In the end, companies like Sonos often play on brand loyalty and the premium tag—creating a form of price “gouging” by regional market. It’s one of those cases where the solution lies with either some regulatory oversight or competition driving down prices, but until then, consumers in certain markets are stuck bearing the inflated cost.

A certain level of markup is required to be able to offer free support forever.

Same in the US so that can’t account for the difference.