Which one should I use?

+Spd is useless on her even though she takes a less hurtful bane.

+HP, -Def is somewhat neutral imo as opposed to taking an HP bane and raising a useless stat.

Neither boon is especially helpful. HP is good in a vacuum, but with -def she doesn't survive any better. It's a bit like fury in reverse. With fury you take three less damage in combat, but with recoil you take 3 more in the end. With +HP -def she has more HP, but she loses it in one hit, and her ability to take the second hit is worse.

+spd could conceivably dodge a double, say, from Deirdre or some slow vengeful fighter if you're using CC. With support it's also conceivable you can force through wary fighter, for example, on Effie -spd. So it's not entirely useless but it won't change much. I'd go for this over +HP -def.