seems like you don't know a whole lot about bikes, so buy from a local bike store, regardless. $500 dollars is a pretty cheap price for an entry level commuter bike. its gonna have bottom barrle compoents, unless you want to spend over a grand. neither of those bikes are "fast bikes." they are for one thing, to get you from A and B. comfort comes second, steel will be smooth and will probably weight 3-5pounds more(weight won't matter to you unless you load it in a truck, hang form the roof, or are climbing mountians) where as the alum won't rust, will be lighter, but you will feel every bump in the road. i'd go for steel for commuting.

if you want to save money buy used. look out in 2nd hand stores like good will or salvation army, garge sales, the local paper or cragist list and learn how to tune up and fix up a bike. tons of websites with video tutorials about how to overhaul headsets, change tires, wraping bartape, ect. imo 500 for a cheap bike from a local bike store is 10times better than any target/walmart bike. often times a lbs will do free tune ups and adjustments for serval years or a lifetime.