Some things to think about:
I like a steel frame because I find them comfortable, durable and affordable. Steel is a little heavier than aluminum, but often gives a more comfortable ride.
If you are commuting 5 miles each way or less and it's not too hilly, consider a single-speed bike. They are low maintenance, simple to operate and cheaper than a similar geared bike. Riding single-speed will make you a stronger rider and you may find it more fun than riding geared; I do and my commute is 25 hilly miles round trip.
Look for a bike that can use wider tires, 28-35 mm. They will help you negotiate badly maintained roads, dirt roads and other hazards and you'll ride more comfortably.