Just think about it for yourself. They are both as dangerous as each other. Cutting down your arm means that its a bigger cut, could be deep, could get a vein or somewhere vital like that.. therefore you bleed more.
Cutting across the wrist means that it could be a very very deep cut, which could reach your artery.. then you could bleed to death in seconds.
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UndergroundKing is ma homie man.. I wanna smoke with him haha