Steel is a great material for bike frames. Unfortunatley, it takes more work, technique, and thus cost to make a great steel frame. And you will not get a great steel frame in a bike that costs $400. What you will get is a steel frame made from straight gauge steel. It may not even be chromoly (CM). If it is CM, it is the absolute bottom of the line chromoly. Back in the early '90's almost all mid level bikes and up were CM steel. Aluminum was almost considered exotic. Now, triple butted CM steel is exotic and aluminum is common place, mostly because it is now cheaper to produce aluminum frames. There is nothing wrong with today's aluminum frames. In fact, there is a lot right with them. New hydroforming and welding techniques are filtering down to even entry level bikes. Aluminum rides as good as steel or anything else with the new production techniques, ie., hydroforming et al. Suspension systems really opened up the widespread use of aluminum for frames, and now with the new hydroforming it is a no brainer. Steel is still great but you will pay a premium for good steel tubing. Ordinary high tensile steel and low level CM are not superior in my opinion. Really expensive triple butted stuff like the Tange, Reynolds, and Columbus are good but you will not find this tubing anymore on bikes that are under $1,000. Most real good steel bikes are $3,000 and up.
Bottom line, don't sweat it that the bike you are looking at is aluminum. It is most likely better than the equivalent cost steel bike.