If you'll just be working with steers up to 1300ish the 7 foot should be enough but you might appreciate that extra foot. Big heifers usually take that extra foot and if you tie and let them step back behind the headgate then it will come in handy. The steel chute is strongest, aluminum lightest and most convenient, and I'm not sure that the hybrid will do you any good, If a calf is bad enough to bend up the aluminum chute I'm not sure that steel frame will help any. In my experience, it takes a pretty crazy calf to bend up the aluminum bars. Also, the padded floor will absorb a little water, but not much and the padding really helps keep them from getting tired and stiff.
So to sum it up, I would buy an 8 foot chute with the padded floor, and steel or aluminum is up to you as I don't know what kind of cattle you will be putting in it, I own one of both though and you have to be a pretty strong person to deal with the steel one solo but two people can handle it easily.
