I have four basses. Two strung with flats and two with rounds. I have played both types of strings in a band setting and they both sound good to me. The flats have a punch thump where the rounds have a crystal clarity.
My perferred string is flatwound, because I like the way they feel, that they make less string noise, and last longer than rounds. But for some stuff, rounds seem to sound better to my ears.
When playing with others, I've received more compliments on my tone when playing flats, particularly from the guitar player, as they tend to complete less in his sonic territory (if that makes any sense).
When you are trying out strings, don't judge them by how they sound standalone. Play along with a backing track or a band and judge them. Flats sound dull when played by themselves, but in a band setting, they are marvelous.