See, if Sammy was still here, she'd have the answer for you!
Steel is VERY heavy, and is great for a kids bike, but not for any kind of serious biker, they would never buy a steel bike (as far as I remember).
Usually aluminum (I think?) Or titanium. I think my dh's GT Mountain bike is Titanium and is worth a hella lot of $$$ I had it shipped here from a shop in Arkansas when we were there visiting. He found it and fell in love with it. I called the owner up later and offered him a fair penny to ship to me so I could surprise him with it.
But then again, my dh used to be a pro in the "sport" of bicycle (or whatever you call it! LOL!!) Anyway, all I know is he has great legs and a NICE rear from it.
Just remember, heavier the bike, the harder it is to ride for any kind of endurance riding.