I was under the impression it was weight. To my knowledge, forged aluminum is harder and more rigid than magnesium. http://www.differencebetween.net/object/difference-between-aluminum-and-magnesium/
Realistically, the weight difference between the two doesn't justify paying the extra money in my opinion. It would literally be impossible to notice the difference once everything is mounted. Just my two cents.
Thanks! I love the look of them. Dying to skate them fully.
I threw on some outdoor wheels and took a few sprints down the street last night. They are super comfortable. Impatiently waiting until friday and saturday to actually play on them.