At Sonos training last week - Sonos head tech said that SOME phone techs with OLDER information would recommend wireless. But the company line is that wired is best. Preexisting network anomalies might cause issues when players are wired - and a fast, easy solution to a hard to track network problem is to go wireless. So some techs take the easy way out and tell you to ditch the wired connection.

I've wired every device I could on every Sonos install I've done - many with Comcast/Xfinity gateways. I've never had to forgo a wired connection for wireless. That might help answer the "rule of thumb" question. Sorry OP, I have no answer for the Comcast gigabit gateway question.