I would not say the One and One-SL would sound deeper in terms of Bass frequency, but obviously two paired speakers would output twice the volume at the same level and provide separation for a much wider stereo sound stage.

So whilst an Era-100 does output stereo audio standalone and deeper bass frequencies, and sounds better than a One/One-SL, because you can’t stereo pair it with your ‘existing’ Sonos One, you are probably best to add a One SL, just for the wider stereo separation. If you want to add more Bass, then you would certainly be better off later adding a Sub-Mini to that stereo pair.