Personally, i would recommend steel (reynolds 853 steel is what to look for i believe). The reason i prefer steel over aluminum and carbon, especially for a commuter is that it does the best job of absorbing the bumps and cracks along the road. Carbon is the worst, with aluminum in the middle. If your going with a steel based frame (hybrids exist with carbon forks, etc.), that usually means going with either Lemond or Bianchi. Obviously with steel your gaining some weight on the bike.
There's really nothing much that seperates a quality Giant, Trek, LeMond, Specialized, and Bianchi. Almost all use the same components (i.e. shimano or campgnolo). The most important thing is fit and comfort. Not all "similar" bikes (in terms of price and parts) from different manufacturers, and even within the same manufacturer, carry the same dimensions. One model may have 8 different sizes available, whereas another model may only have 6 different sizes available. Its highly recommended to find an informative bike shop that will help fit you in your proper size, and from there choose a bike from a particular manufacturer.