It will be several weeks before these are shipped to us mere mortals, you may find a quicker comparison in the ‘press’, who may get access earlier than the rest of us. 

I’m a patient guy.  Going to wait for the mortals.  I’m not sure if I will replace my Beam.  I’m faced with location issue.  The Beam fits perfect.  The Arc is 19” longer.   I’ll submit a pic on this thread soon.

IMO opinion as a mere mortal I don’t think there is a comparison between the Arc and Beam for sound reproductions'.  Dolby Atmos and Dolby Digital plus aside; if one were to play material at just DD5.1 the Arc will blow the Beam away.

The Beam even with it’s advanced technology struggled against the Playbar (and I own both) in terms of sheer cinematic sound reproduction.  I don’t believe Sonos would introduce the Arc with lessor performance than the Playbar which IMO trumped the Beam overall as a listening experience (expressed in the simplest terms). 

Do you have to drive a race car to know that your stock Mustang GT would most likely fall short in a head-to-head straight line race. I use the term straight line because race cars are setup for left turns and would most likely struggle against a stock Mustang GT in a race with right turns. Although in the end the race car would most likely prevail.

However, if space is an issue then sound be dammed...stick with the Beam.  JMTC  on the subject.

I have the Beam and plan on purchasing the Arc and can post my findings and opinions between the two but I agree with @AjTrek1, it's pretty obvious without 3ven testing that the Arc will easily beat the Beam.

I have purchased several 4K blue rays to test out the beam set up vs the arc. I feel that the arc will blow it away.