It is different. Every experience I've had with rounds vs flats on a fretless is you always get a more defined, sustained "mwah" from rounds, whereas flats give a much more muffled, softer sound. This is because round wounds produce more definition and harmonic overtones which translate to a more obvious "mwah".
For the record, Jaco used rounds.
Yeah but paul mccartney played flats and tapes, and he smokes jaco for technical and fast and complex playing. I have heard him actually play more than just the root note before under a chord.

There is simply zero truth to that statement.