interestingly enough, my first bike was a 92 Scott chenango, and I now have a 2002 rockhopper frame. I'm about 220lbs fyi.
The most noticable thing you'll notice is the change in ride, the AL bike being harsher. Also, aluminum has a much shorter life span since its stiffer, but should perform better for us big guys than a noodly steel bike.
I would prefer steel for the longevity and the confortable ride, but I ride AL for the stiffness. Just dont expect it to last forever.