FCorrin question

I’ve been wondering the same thing

+SPD is usually the best option to prevent doubles. Keep both for different situations. It's ultimately your choice.

+Atk is also quite good since it receives a superboon, making her atk from poor to actually decent

I'd go with +Spd because you can make her a dragon Beruka - utilizing her good defense to deal intense burst damage, except Corrin can actually avoid and potentially make doubles because her speed won't be garbage.