Aluminum is about a third the stiffness of steel given the same section. And it weighs about a third as much as steel.
What that means is basically you need about the same weight of aluminum to get the stiffness of steel.
The proof in the final assembly is testing the, both. Weighing them only gives you a peak that maybe one is not as stiff as you may think.
Another property of aluminum is fatigue strength. It doesn't have much until you make the leap into the 70-series. Welding metals makes for all sorts of assumptions of what is going on.
Those old bonded aluminum French frames felt pretty whippy to my old racers legs back in the day. I stayed away.
The Cannondales are/were large diameter and said to be post-weld heat treated. They sure were stiff.
I think it was Lehigh or Pitt that built a bicycle torture rack that would flex the BB side to side to evaluate fatigue and try to correlate the analysis with this testing. As I recall it worked pretty well. This back before we had fancy NASTRAN to mesh structures.
It took me a long, long time to become comfortable enough with the manufacturing techniques and materials to break loose the money for a CF framed bike.
I love it as a road machine. It is comfortable yet a little stiff of ride. No perceptible sway when climbing, and light. My old steel hand made Italian steed feels like a sled in comparison and a wobbling, flexiflyer at that.