Well, +burst is still useful for a union whose strategy is people jumping in during burst time, unleash their burst, and don't care what happens afterwards.

...but for the union I'm in, now I'm strongly leaning towards double atk or triple atk over burst.

Double atk does have a higher base rate compared to triple, so if the bonus is multiplying by their base rates (as opposed to adding some flat bonus), then maybe +double is a lower ceiling/higher floor type choice?