IMO no comparison steel to aluminum. My first bike was steel and even with an aluminum fork a way better ride than my aluminum commuter with a carbon fork. Weight? Rubbish you aren't aspiring to be some skinny climbing racer are you? I don't worry that my Cannondale is heavier than your average bike because it has a triple and 105, she rides well and loves me, so I love her. 
Stiffness- I never really understood until I got my commuter. When I am on my Cannondale and get out of the saddle it is firm under me, if I am on a small enough hill to really put power in the climb I can totally feel it and the bike feels firm. I joke with my husband that I tell her to climb and she goes "yes ma'am let's go!" My aluminum commuter is not stiff. I get out of the saddle to climb and I feel like I am walking on the old floor mat from the days of gymnastics! That is an extreme comparison but I seriously feel like the bottom bracket and bike sag under the force of my pedaling.
Amanda
2011 Specialized Epic Comp 29er | Specialized Phenom | "Marie Laveau"
2007 Cannondale Synapse Carbon Road | Selle Italia Lady Gel Flow | "Miranda"
You don't have to be great to get started, but you do have to get started to be great. -Lee J. Colan