I would personally got with the +Spd version...but it depends on how you plan to build him... if he has constant access to buffs, +Atk works fine, since he will reach 46-50 Spd, but otherwise the +Spd would be better in my opinion
Up
0
Down
Unless someone changes his base kit, Roy buffs himself with his C skill, so he has basically +12 atk/spd as long as he initiates his turn next to someone other than beasts and dragons. That should be enough spd to double most units.