If your muting is poor it doesn't matter what about tone or feel, bad muting technique through a few hundred watts on rounds sticks out like a sore thumb. You can get away with a little more on flats.
I have a dedicated bass with flats, you don't get the extraneous overtones you get with rounds, for some players that is going to appeal.
I like the flats sound, but using them constantly would be very vanilla for what I like to play.