I'd expect the Arcam Solo range to have better DAC's, opamps, capacitors, etc than the Sonos. In that respect I would imagine that it would be better to connect digitally from the Sonos to the Arcam. Try both and let your ears be the judge.
Alternatively you could consider an external DAC such as Cambridge Audio or Beresford for music purposes. Either of these will produce good results out of the box, but can be upgraded as funds allow to produce a sound far beyond the cost of the upgrade.