Both don't scale very well, but bloodlust gives you more sustain and overall potential damage.
Spin really only needs one level, it's base damage doesn't increase by much, and ratios don't change, only cd. In all honesty maxing shout is more effective than spin. I believe it reduces enemy ad by 80 at max level, regardless of back turned or not. I've seen a trynd go 1 point in everything, put a second in q, then max shout instantly. He was against a riven, so his early was meh, but then riven had either (can't remember too well) 60 or 80 ad at every engage because she only had a brutalizer and vamp scepter at around level 10. It was pretty hilarious to see it.