Fairly important, even more so if you plan on using Apple TV. 

HDMI 2.1 contains eARC. Or vice versa.

Without HDMI 2.1 in your TV, you likely won’t get any Atmos content, which requires both ARC and a minimum of Dolby Digital Plus. 

Ron,

FYI  these 2 threads regarding issues with the ARC caveat emptor

https://en.community.sonos.com/home-theater-228993/sonos-arc-muddy-bottoming-out-bass-6842379

https://en.community.sonos.com/troubleshooting-228999/sonos-arc-post-build-58178180-6843904

Thank you!

@Airgetlam 

read both threads and though I cannot say I completely follow all the terminology mentioned there, would you say that there are many bugs in the Arc, making it an inferior product?

There are an unknown number of users having issues. First attempt by Sonos didn't fix it according to some. You may or may not have issues or listen at a volume to hear it. No idea if there are many or any bugs. But thought you should be aware of the issues some are having to inform your decision. 

I have two Arcs, and am having no issues whatsoever. I suspect many of the issues being experienced are due to the TV sets, or the devices feeding the TV sets. Having eARC removes much of that as a potential issue, but it’s clear that some folks are having some sort of potential software issue, which I am not.
 

Having been an owner of Sonos devices for a long time, as well as being familiar with software development, and having seen Sonos address these rare type of issues in past (only one time that I can recall, offhand), I have every confidence that they’ll be able to track down what those folks are experiencing, figure out if it is an issue in Sonos’ software, and if so, resolve it as soon as possible. 

Do you use a sub with your arcs? 

Indeed, I do. And both also have a pair of PLAY:1s as surrounds. 

So Ron FYI the arc sound issue/defect/bug appears to be present primarily with folks who do not own subs.