Steel is roughly three times denser than aluminium, but you need to go about 50% thicker on sections and sheet/plate to get the same strength.
Even taking that into account you should save at least one third in weight.
The real beauty of ally is its corrosion resistance, for example I almost never use box section steel because you can't coat/paint the inside of the section. Box section ally is great though, and if you ever need to alter your aluminium fabrication you don't need to worry about stripping the paint before welding and recoating when finished.