Round wounds sound brighter and have more bite which can tend to growl or Buzz down near the nut when really digging in. Sometimes a slighty higher action may be required to sort things out.
Flat wounds sound more mellow to dull in comparison but smoother on the fingers and generally more forgiving.
I only have flats on a fretless to protect the fret board. It would take rounds but they can leave more tell tales signs of wear.

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