Many factors play in your decision, such as, but not limited to how much and what type of riding you will do, the quality of the streets/bike paths on which you will ride, and how large you are.
I noticed some people were recommending staying away from aluminum based on their experiences with Cannondales. I can't speak to the lower end but the higher end Cannondales are superb rides, especially for larger riders like myself, 6'3", 240lbs.
Go to
http://www.roadbikerider.com, click on product reviews, and then click on the Cannondale R5000 road test. The reviewer loved the bike, which has a CAAD7 aluminum frame. This is the same frame as the R1000, R2000, etc.
Another thing to consider is how well are you going to take of your new bike? Any little chip on a steel bike requires immediate attention to keep it from rusting. I have several chips and nicks on my aluminum Trek and not to worry. No rust. I can touch up the paint when I get around to it.
Steel will definately be a more comfortable ride and the Lemond Alpe D'Huez, Buenos Aires and Zurich are super bikes. The 2004 Zurich is, I believe, a combination of carbon and steel. As to the Trek and the Klein, either would be an excellent choice.
But don't dismiss higher end aluminum and Cannondale's out of hand. My next bike will, more than likely, be an R1000, unless something drastic happens between now and when I purchase.