Flats have a very vocal cult following online. Generally it’s a minority of players, but they do love them.
The sound is very different, as is the feel. Some like the feel of rounds better, some like flats better.
Here is the distribution here on Bassbuzz, which in my experience holds up with others I know:
Generally it’s about what kind of sound you’re after. Flats are good for a thumpy motown, old school R&B vibe. Rock and more modern styles typically favor rounds. There’s exceptions of course.
Really it’s all about what sounds and feels good to you. Because:
this will absolutely happen when you ask others their preferences. Don’t let anyone talk you in to either, both are equally good for new players. What matters is the sound and feel for you.