+spd -def AF! Corrin

Neither, if you ask me. +spd is amazing, and -def I think you will definitely feel with her HP being thin. GamePress tends to like to write their IV guides in an exaggerated manner like "oh his res is terrible if you drop it it won't matter at all" - like, no. As an example, M.Azurcorrin has 24 res and that's better than neutral Zelgius plus there's a superboon available and he will tank on his res if he has a superboon and he uses noontime or possibly sol and definitely if he uses shield pulse. In general, very few banes other than spd are without consequences if the stat begins in the mid 20s, and -spd makes a difference on Dorcas, as I know from experience.

Don't let the -def scare you. You have a crazy powerful unit. Work with the bane.