+Def - Res or +Atk - Spd Summer Noire

Depends, really. +Atk/-Spd Noire can still run a Desperation set, and I'm sure she'd appreciate the extra attack from the boon, but +Def/-Res has a boon that doesn't touch any of her important stats.

I'd say sacrifice the +Atk -Spd Noire. Losing speed is a huge setback, especially on a potential speedy archer like... well, S!Noire. With neutral Spd, she can hit 40 Spd (with her base kit and weapon refine), which is really good speed. A Spd bane sets her back at 37... decent Spd, but not high enough to take out some of the speedy heroes in the game.

All in all, it's always your choice of how you want to handle this delimma. Hope this helps provide some form of thinking!