Usually, the 640 has a somewhat smaller body, in proportion to the reduced scale length. A good luthier can compensate for this so that there is no essential difference. Otherwise, you might notice some reduced bass or a bit less sustain, etc.
The shorter string length--we're only talking here of 10 mm--will not have any effect on sound quality, to my knowledge. However, if you were using lower tension strings, you might have to go to the next tension level. Shorter strings have less mass which requires less tension to reach a given pitch.
While going from 650 to 640 MIGHT help improve playability, by itself I think it has only a small influence on playability. Neck shape can have a greater influence, with Delcampers seeming to prefer the deeper, narrower C shape over the wider, flatter D shape for playability.