i think flat wounds just last longer in general because there's less of a dynamic range to begin with, compared to rounds. i know a set of flats will sound brand new for months whereas a set of rounds will last for about a week before they start to sound dead. look at upright strings, for example. i know guys that haven't changed those in literally decades, and they still sound great (not counting gut strings. i don't really know anything about those).
i know just about everyone will tell you flats are better with a fretless, but i never liked the combination. if you're not a bass player, though, it's the easy way to go. and i can second the chromes suggestion, and la bella makes nice flats, too.