I won't argue on the first or last point. But there are some alloys of aluminum that are as strong as some alloys of steel. Aluminum is also generally easier to work, unless you're trying to weld it. But steel is much stiffer. That is for a given load, a steel beam will deflect less than an identical aluminum beam. Of course the steel will weigh three times as much.

The key here is choosing the right material for the right reason. If weight doesn't matter (you're way under the limit) or stiffness does, then look at steel. But most of the time aluminum will probably be the better choice, because with careful design you can meet strength and stiffness requirements and still weigh less than a steel structure. You also need to pay close attention to which alloy you are using. Different alloys of the same basic material can have quite different properties.

we used a combination of bosch and maytec? aluminum extrusion.

Why should you care you ask?

we didn't make a single repair all season to our frame.... or to our robot for that matter.

I'd say that you'll do just fine with some 1" extrusion. It's cheap and light, and it is easy to assemble/disassemble. Allthough steel is nice, it'll be easy to get along with aluminum.

This is a good point, however if you choose steel, as also you said, that increase the weight drastically. I would think if you were only ramming into things, with a motor and a frame, steel would be ok, but in my past experiences, even with Al frames, weight is a BIG consideration.
Ah yes, now i remembered what i was planning on doing, weighing equal lengths of 80/20 and 1x1 Al tubing. Would anyone happen to know the weight or density of these materials?

I want to say one foot of 10/10 is 4 lbs, but thats just what I think I remember seeing.

That's basically the decision we came to in 2002. We had a simple, small robot. With aluminum, we'd probably have ended up about 30 lbs under the weight limit. Our goal-grabbing arms also needed to have no bending and distortion at all when they got hit, which, given their small size, I don't think could have worked in aluminum. We'd have been straightening pieces all day.

Welded Chromolly* steel tubing for this year (2003)!