The Era 300s as surrounds will give you a wider and taller rear soundstage because of the side- and upward-firing drivers especially when playing Dolby Atmos audio.

Thanks. 

When i called Sonos tech support i was advised that the ERA 100s would be better than the ERA 300s as surrounds with an Arc. 

Not sure I’d agree with that statement from tech support. I’d tend to go with @GuitarSuperstar’s comments, as being more knowledgeable. 

I just switched out a pair of ONE SL’s with ERA 300’s as my surrounds, matched with an Arc and a Sub, the improvement is significant. However, my original set up sounded great as well. So, I’m sure that the ERA 100’s will do more than suffice. Before I added the ERA 300’s as surrounds, I set them up as a stereo pair. I was blown away. I was able to easily compare The ERA 300’s with a pair of Sonos 5’s by switching rooms during songs and, in my opinion, the ERA 300’s won that battle easily. I was concerned that the spacial audio would sound gimmicky. It didn’t. It filled the room with music and actually sounded more natural. If hadn’t already invested in the 5’s, I would buy the ERA’s 300 as a stereo pair and the ERA 100’s as surrounds. 

Sorry for the long response, but you can’t go wrong either way. 

I have had similar reactions. I replaced my One SLs with ERA 300s in my HT setup and love them. I watch a lot of Atmos content via 4k BluRays and their overall effect with that content is noticeable to me.  I have my surround levels and height levels both at +4 as a default, which is similar to what I tended to do with the One SLs and the Ones before them.  I also have two ERA 300s in my living room, which replaced two Fives. I had some doubts when I ordered them given the cost, but it has worked out well. My living room is narrow and no matter where you sat, the stereo image was always off with the Fives. With the ERA’s, its more balanced and immersive no matter where I am in the room and even nearby in the kitchen.  But since their main goal is to deliver Atmos content, which is definitely different than a traditional stereo image, they likely won’t make much sense for those that value the traditional stereo sound.