Thread:DeathByStaccato/@comment-27960787-20161020061901/@comment-27076522-20161020112148

https://www.reddit.com/r/bravefrontier/comments/51sbov/jpbf_new_units_98/d7embav

I'll let you read that as it should explain quite a bit in terms of which are better.

In terms of the difference, you can think of OD fill rate like the BC fill rate buff - it makes the gauge fill up faster.

You can then think of flat OD% fill like the BC fill on units like Melord - it simply fills the gauge, although OD% fill increases in accordance with the cost of the OD gauge.