Strength, stability, with as little weight as possible. I prefer Steel, as they will stay stable longer than Aluminum. 1 1/2" steel square tubing makes a great chute.
We built one out of 2" square stock x 1/4" and that was heavy, that one stays home now in the barn. Built the next one out of 1 1/2" square stock, much better, make sure you have a decent set of wheels under it. Good Luck.