which brave lyn should i keep?

-Def+Spd, definitely. That's basically her ideal IV for all of her builds.

+atk/-hp is better than +spd/-def

With swift sparrow, she will have 39spd when attacking and 54atk.
With some buffs, especially cavalry buffs on a cavalry team, the speed is high enough to counter almost every unit. Hell, you can achieve 49spd and 64atk with some luck on a cavalry team.

+spd bonus doesn't matter at that point, it would be overkill on an already fast unit.

+spd is better for arena, +atk if you want to use her for pvm

+spd for sure. Works fantastic with every one of her sets, even the Brave Bow+ one.

Depends on the build, +atk is generaly better for brave bow while +spd is ideal for firesweep(and my personal choice)