I know that flats have less surface noise and a more mellow tone, perhaps more fundamental and less harmonics. Can the same result be achieved with roundwound strings by turning down the tone control?
I suppose one other thing to take into account is the different envelope shape with the flats having more of the thump, i.e. a softer transient, and less sustain.
I'm expecting delivery of a short scale Talman bass and there seems to be a long standing fashion for equipping short scale bass guitars with flats. My only experience of flats was an acoustic bass guitar fitted with black nylon strings and that produced a passing resemblance to a double bass.