Simonsez, I'm looking at the same kind of bikes myself. Those are commonly suggested as general-purpose bikes, good for both road and (light) trail riding, so maybe that's what you're after. Steel is indeed commonly said to give a smoother ride than the other materials. Aluminum is commonly said to be harsher.

I think it was on the Soma website where I read that aluminum is actually cheaper than high-quality cromoly steel. I've also read that another advantage of steel is that almost anyone can weld it if you need a major repair. But I think it's a mistake to get all purist about steel and consider it the only "real" option. There are lots of good bikes made from other materials.

Anyway, there are some decisions that require more than mere abstract thought. The best idea is to ride as many different bikes as possible, and decide for yourself based on what feels good to you.