With a Brave build, +Spd on Rebecca doesn’t do much. Going from 38 speed to 41 speed means she can double enemies floating between 33-36. These units are frequently bulky, such as fast armors, or standard +Spd infantry dragons like Fae/Nowi. These same units will easily reach a fair bit of defense, and honestly might take 0 damage from each hit.
For instance, the typical +10 +Spd, Def Refined Nowi with 34 speed has 38 defense on enemy phase (prior to any support or the commonly seen DD seal). She’d take negligible damage from even a Quad; moonbow would do damage, but only like an additional 12.
Armors like SB Zelgius, uh, have even more defense than this.
So that makes Rebecca only suited to kill squishy units, like those around 20 defense. If you’re aiming to kill them, you don’t need the Desperation Quad. +Atk would benefit her way more, since that’s effectively 6 additional damage per attack.
I would recommend +Atk for a Brave build. Some people opt for Chill over Desperation, but it becomes more about team comp at that point I suppose.
Have you considered Firesweep Bow as well? Rebecca can hit 46 Speed with it.