It's funny, if you asked me a year ago I'd say Low every time. Now, I'd say I was a fool.
Low Mode is quite good as a default state to be in. Great stability, easy to control, and works immensely well for Attack especially. Guilty, Xiphoid, and Ultimate have no reason to ever consider leaving Low Mode unless they're actually struggling to make clean contact with the opponent, and that's particularly rare. Guilty especially has such a strong vertical reach that I don't think there's a single bey it can't hit in Low Mode. Its shorter height makes Attack in general stable enough to land more powerful blows.
Thing is, Stamina doesn't work like Attack does.
It may seem odd to even suggest that High Mode would be better, but the truth is it really is better for many pure Stamina combos if you're properly balance tuned. If you're badly tuned, going into High Mode is asking to shimmy and shake your Stamina away, and that could be the difference between victory and catching the tornado ridge and self-KOing. Still, that's not to say that it's bad if you are decently tuned. In fact if you're tuned you're going to be the one on top, and for Burst that matters. In MFB things destabilize other things by hitting upwards to them from below, but Burst seems to work on a contrary principle: hitting things down is more likely to destabilize them. This is especially noticeable for high LAD drivers with poor stability like Bearing', which is easily knocked over by High Mode.
Let's look at World for a moment. It's made a bit of a comeback lately on drivers like Drift and Bearing Drift by using its powerful rounded shape to repel attacks and equalize enough to stay in the game. In fact, its thin edge is so good at doing this that it can often beat low mode DB in opposite spin. It's a terrifying combo when it hits its mark... unless your opponent realizes that it's an opposite spin match and goes to High Mode. World can't destabilize the taller opponent as much, and suddenly it's the one being forced into the ground and at the disadvantage. It's funny, but High Mode Dyna+F is a lot better than Low Mode Dyna+F against left spin World, and that's not the only matchup where High Mode is actually better for your LAD.
Do keep in mind that not every driver will behave the same in High Mode. Any imbalance will make it hard to control (and to this note I don't suggest using Dragon or Valkyrie up there if you can, those two tend to be pretty close to impossible to balance tune well enough), and there is definitely such a thing as being too tall. High Wave' and Bearing Mobius, for instance, really should stay in Low Mode the vast majority of the time. Most everything else you'd use for stamina or LAD can usually use High Mode fine, and usually to great effect.
All in all Low Mode for Attack, High Mode may legitimately be better for most Stamina matches despite conventional wisdom.
Low Mode is quite good as a default state to be in. Great stability, easy to control, and works immensely well for Attack especially. Guilty, Xiphoid, and Ultimate have no reason to ever consider leaving Low Mode unless they're actually struggling to make clean contact with the opponent, and that's particularly rare. Guilty especially has such a strong vertical reach that I don't think there's a single bey it can't hit in Low Mode. Its shorter height makes Attack in general stable enough to land more powerful blows.
Thing is, Stamina doesn't work like Attack does.
It may seem odd to even suggest that High Mode would be better, but the truth is it really is better for many pure Stamina combos if you're properly balance tuned. If you're badly tuned, going into High Mode is asking to shimmy and shake your Stamina away, and that could be the difference between victory and catching the tornado ridge and self-KOing. Still, that's not to say that it's bad if you are decently tuned. In fact if you're tuned you're going to be the one on top, and for Burst that matters. In MFB things destabilize other things by hitting upwards to them from below, but Burst seems to work on a contrary principle: hitting things down is more likely to destabilize them. This is especially noticeable for high LAD drivers with poor stability like Bearing', which is easily knocked over by High Mode.
Let's look at World for a moment. It's made a bit of a comeback lately on drivers like Drift and Bearing Drift by using its powerful rounded shape to repel attacks and equalize enough to stay in the game. In fact, its thin edge is so good at doing this that it can often beat low mode DB in opposite spin. It's a terrifying combo when it hits its mark... unless your opponent realizes that it's an opposite spin match and goes to High Mode. World can't destabilize the taller opponent as much, and suddenly it's the one being forced into the ground and at the disadvantage. It's funny, but High Mode Dyna+F is a lot better than Low Mode Dyna+F against left spin World, and that's not the only matchup where High Mode is actually better for your LAD.
Do keep in mind that not every driver will behave the same in High Mode. Any imbalance will make it hard to control (and to this note I don't suggest using Dragon or Valkyrie up there if you can, those two tend to be pretty close to impossible to balance tune well enough), and there is definitely such a thing as being too tall. High Wave' and Bearing Mobius, for instance, really should stay in Low Mode the vast majority of the time. Most everything else you'd use for stamina or LAD can usually use High Mode fine, and usually to great effect.
All in all Low Mode for Attack, High Mode may legitimately be better for most Stamina matches despite conventional wisdom.