i'm around 260, i've got a 1979 Red K Steel Frame Fixed Gear Track bike. i love it, it's the one i ride 99% of the time, i've also got an 80's Cilo steel frame road bike, love that too. i wouldn't call it flex with steel, it's more compliant so it absorbs vibration more. I've also got an early 90's Trek 1200, it's an alu frame and it isn't as comfortable as the steel frames but it isn't uncomfortable either. And last i've got a 96 Trek 2120, which is an aluminum rear triangle with carbon main tubes in the front triangle, great bike but i just don't ride gears anymore really(unless it's a really long event). And i've ridden titanium frames, full carbon bikes as well as front alu. with a rear carbon triangle, none of them were flexy, they all felt great. I just prefer steel to the other materials because i dont' think that any bike in the other materials(that i can afford) is as pretty as a nice lugged steel frame. They provide a great ride and they have more character than any other bike(in my opinion.)
so is there a difference between the materials? yes there definitely is, however it isn't as big as it may be in cars(i.e.- the difference between a caddy and corvette). Also it defintely has to do with the quality of each material as well as the way it was built. Yeah if you gave me crappy steel tubing and gave it to me to weld in my apartment it'll probably fall apart, but that goes the same for alum. Also if you gave me carbon to fab a bike it would end up being a shotty bike. Assuming the build/material quality is high you'll be doing fine with any material.
It's similar with wheels, the person building the wheel counts as much as the materials used to build it.
Cliff's Notes- By a crappy bike in any material and you're going to get a crappy bike, get a good bike in any material and you're getting a good bike.
sorry for the long post, hope it helped though.