I was wondering the same thing so I Google'd it and found the answer at the official Iron Man website http://ironmanmovie.marvel.com/ The site takes a lot of time to load each page, but if you enter the site (at bottom of home page) you can click on each format (single, 2 disc, 2 disc blue ray) to see what is included. It wouldn't let me highlight and cut and paste or I would have just posted it here.
I took PL's advice and checked out the Iron Man website. While the single disc has 20 minutes of deleted and extended scenes, the two-disc set (DVD) includes a making-of feature, a six-part feature on the history of the characters, a visual effects feature, an actors' rehearsal feature, still galleries, and supposedly other stuff. The Blu-Ray has all of that plus a virtual explorer of the armor (sounds cool) and movie quiz feature, and other things. If you like a lot of behind-the-scenes stuff, the two-disc set is worth the extra dollars. I happen to like those features, so I'm spending the money! The site also mentioned that the features are in 1080p hi-def.