As best as I can imitate a materials scientist/engineer, yes, steel is the stiffer material. However, to keep the weight down, the steel tubes used on bikes are quite thin and also pretty small in diameter. Since aluminum is a much lighter material, you can build oversized (in diameter) tubes that weigh less, but are potentially stiffer than the same-sized (length) frame tubes would be in steel. Hope that made sense on paper...it did in my head, I think.