My personal preference leans towards longer rounds, for simple reason of drama. If a round is 6 seconds, it doesn't matter whether you take 5 or 10, you're still done in under a minute.
If they take 30 seconds, those extra 5 rounds is 2 and a half minutes. That could make more of a difference. And seems more fitting to high fantasy, not every fight is blink-and-you-miss-it.
For instance, if you need to stop a ritual an extra 2, or (with 1 minute rounds) 5 minutes is going to make a big difference. An extra 30 seconds? Not so much. And if you want the ritual to be occurring, with a chance of finishing, during the combat, it's easier to justify why they turn up at the right time if rounds are longer.