I've owned steel and aluminum. I cannot speak of longevity on either. The steel bikes were in south Georgia. Rust ate them. I ride aluminum now but that is by happenstance. I haven't had it long enough to know its durability. What I can tell you from intensive experience in other fields is that I'd rather get my workouts by overcoming terrain rather than overcoming my equipment. In backpacking I spend a lot of extra money making sure I have top quality equipment that weighs as little as possible. I expect I will do the same with bikes as my experience builds. If this is so, I will end up with a carbon or a ti bike for commuting.Why do everything in your power to be fit and trim then decide to push an extra 20 pounds of bicycle around every day instead of using that energy to cover more real estate?! My current bike doesn't even have a luggage rack because I have a system where I only re-supply at work one day a week so I do that with a backpack instead of lugging add-ons around the rest of the week.
I love my brisk commutes.