Which Roy should I invest in?

I would say +atk/-hp is better than +def/-spd for certain. For a hero like Roy, going from 31 spd to 27 spd is a huge hit to his ability to soak dmg, making the +def mostly irrelevant. He doesn't have high enough speed to double most enemies, but at 31 base, he can get to a decent 34 with Fury and avoid doubles from <39 speed heros.

Ofcourse, the ideal boon for him is speed, because then he can avoid doubles from <42 speed which is quite good, and combined with Binding Blade, Fury, and his not-bad def/res, make him quite survivable.

So if it were me, I'd wait until I got a +spd to promote to 5*, and use the +atk as a 4 star until then.

Definitely +Atk-HP. This is because Roy can take a hit on HP but can’t on spd because he’ll be doubled by more units and it isn’t really worth the +def. Also you can merge them together to make an even better Roy. (-HP easily fixed by +3 HP sacred seal.)