The reason the Stage is so expensive is because it's very difficult to achieve extremely pretty design (take a look at it in smoked oak color) and extremely well sound at the same time.
Summary
Looking at it from performance, capability and sound quality standpoints, both soundbars are top notch and comparable.
In short, the staggering price tag of the Beosound Stage would absolutely NOT worth it over the Sonos Arc... Unless you're aware that you're paying for its looks, and not for its technology.
So...
Unless the appearance of your soundbar is vital to you, or a few thousand bucks mean close to nothing to you... then I'd recommend getting the Arc over the Stage my eyes closed.
Design
Design is the biggest only practical factor for the price difference between the two. So it makes sense to start off in here.
The frame of the Stage extends to the seamless integration of the control buttons while the contrast between the aluminum and the fabric pops beautifully due to the deliberate gap between the two.
The overall appearance is of a stately, premium product that makes its presence felt inside a home. This is not a product that blends in. It is striking and beautifully so.
The Arc, in contrast, feels somewhat generic in its appearance (at the risk of sounding harsh). While the matte finish is forgiving, the only available colors are black and white.
The materials do not give you anything close to the organic feel of the Beosound and make it look like exactly what it is: a speaker. And nothing more.
On the plus side, it is more of a ‘wallflower’ than its contender. It does a great job of blending into pretty much any room. So if you can’t be bothered planning your interiors around your home theater system, this might be closer to what you need.
Sound
Let me tell you something right off the bat.
The sound quality performance you'll end up getting from a speaker will always depend on your room acoustics - particularly room dimensions and speaker positions.
The impact of the combination of these two is actually so strong that in most cases, it doesn't even make sense to utter a single word on sound quality without speaking of them.
This is also why it's not unusual to see completely different reviews of the same speaker.
In one case the speaker might be placed in a sweet spot inside the room and hence the user might be satisfied. In other cases the same speaker might be ill placed and hence user might even have returned it.
The point most people miss here is that it mostly isn't even about the engineering behind the speaker itself. It's about where you place the speaker inside which room.
So, in order to solve this problem, I've partnered with Acoustics and Audio Engineering PhD Andrea Cicero from AC Acustica and created Soundton - a simple, 2D, browser accessible online speaker placement calculator.
With Soundton, now there's a way to figure the sound quality of most speakers before you buy them.
Read more about its working principles at soundton.com/documentation/.
The end colormap provides you the locations with the best (green) and worst (red) acoustics.