I'm a strong advocate of aluminum; I always put my two bits in in the aluminum vs. steel debate...

I think aluminum gets a bad rap on the durability and ride quality fronts - I believe that aluminum bikes that are well-designed can be more durable than their steel counterparts, and that ride quality is dominated by the tires and seat, rather than the frame material.

Lighter never hurts, even if you're not racing.

Aluminum won't rust, if you have to store the bike outside, or if you ride in the rain.

A major benefit of steel is that you can have braze-ons added for fenders, racks, etc. And the bikes that come/came stock with those braze-ons are typically steel anyway. Also, if you have to resort to clamping stuff on, steel frames may be able to take more clamping force without damage, and are more likely to have standard tube sizes.

A 1980's touring frame (naturally, these are steel) would be my pick for a good commuter bike... my commute is long, though, and I need an efficient bike.