My hair is too short to need a bun, but here's what I hear from my partners and teammates and coaches that do.
Look-wise, its a matter of preference, mostly. But the height of your bun can greatly change the way your head-weight is organized, and a higher bun generally requires more posture discipline (especially if you have unusually long and heavy hair) So part of the decision on whether to do a low bun or a high one should include which orientation allows you, personally, to control your head-weight better--and make sure you practice sometimes with your hair in the same place!
Examples:
My competition partner prefers a mid-high bun in order to lengthen her vertical line. She has *extremely* long hair (well below her waist), so she can't make it too high or she starts to feel neck strain due to the weight and leverage, in Standard. But her posture discipline is good enough to control a higher bun. In Latin-only events, she wears her hair so high it is practically a topknot.
On my side, I can actually feel the difference in our frame when she ties her hair at different heights. It is that significant. Probably because her hair is, by her own admission, ridiculous.
My wife, on the other hand prefers a low bun. She feels her neckline already looks pretty long without help, and she doesn't feel that confident about managing her head-weight. The low bun basically just keeps her hair out of her way.
Plenty of backcomb and hairspray and I have a different look that works for me. From that point forward I select my hair and dresses. I know what compliments my face and figure.. Sorry, not listening to a young man who has never had to dress a curvy body.