[Awrya]

From my view, DD and BG are about equal. The most obvious use of DD is to halve the opponents strength and take it for yourself, with BG you boost your power and use that to dominate your opponent. But both Ise and Vali demonstrated creative ways of using that power: transferring BG's power to the DD wings and cause an overflow (Ise) or use DD to halve magic attacks to literally nothing (Ise&Vali).

There is one major difference between normal SG mode and BB mode for both, DD gains the additional ability Half Dimension, which has to be used strategically to maximise the effect. BG's ability is maximum boost, reaching the maximum number of boosts in an instant.

In SG mode I'd say DD dominates BG, since BG has a charging time while it is unknown whether DD has one as well. If DD halves your opponents strength every 10 seconds like BG boosts, BG will never go beyond the second boost if both are activated at the same time.

In BB mode it solely depends on who can produce results in the shortest time, as the abilities eat away your BB time.

Edit: just noticed this whole conversation would be better off in Speculations&Theories thread...
[Mod Edit: Posts moved.]