I’m not sure you understand the Sonos paradigm for Home Theater. With a Sonos sound bar, you always get the front right, front center, and front left channels, in a single enclosure. You can not add additional right and left speakers to this configuration. The only speakers that can be added to a Sonos sound bar for home theater use are up to two subs, and a pair of surround speakers. If you use an Amp to drive them, your surround speakers must be third party passive speakers.
You could instead use the Sonos Amp to drive your own pair of speakers in the front of your home theater, and the would create an interpolated center channel. You could add a third party Sub, or, as above, up to two Sonos subs, and either a pair of Sonos speakers as surrounds, or a second Amp that would drive a pair of third party speakers as surrounds.
Sonos does not (unfortunately) offer any opportunity to have three separate speakers in front. You get those three disparate channels enclosed in one box in the sound bar, or you can have two of your own speakers, but not three.
I found the sound quality of the Arc to be superior to the PLAYBAR, perhaps because of the extra width, even before I tested out the Atmos aspect. Given that only a limited amount of content is available in Atmos, I wouldn’t consider that as a primary reason for choosing, or not choosing the Arc.
