In my opinion, threefeatures weigh against an aluminum frame
1) Fatigue limit. Aluminum has no fatigue limit. That is, after a number of cycles it WILL fail. Imagine the Golden Gate bridge made of aluminum, and a single fly lands on it, then takes off again, then lands again. Eventually, over time the bridge will fatigue fail. Most probably not in our lifetime, but it will fail. (No it didn't say how big the fly was... but that's irrelevant.) Fatigue life is based on the level of stress the material sees. Do you know what the stress will be in your aluminum structure,and repetition frequency? To quote Dirty Harry- Feel lucky punk?
Steel, in contrast, does have a fatigue limit. If stresses are kept below a certain level, the structure will never fail in fatigue
2) Elastic modulus. Aluminum is 1/3 as stiff as a similiar cross section of steel. Sure,you can upsize the tubing the recover the structure's stiffness,
3) Ease (or lack thereof) of use. If using MIG, you need either a push-pull system w/ teflon cable liners or a spool gun. Use Argon rather than C25. If using TIG, the system must be capable of A/C welding current. Many economy systems aren't. Also need to know alloy composition of the base and fill metals. As a ferinstance, 6xxx series aluminums are prone to silicone nodule cracking when the weld refreezes, so typically something like 4047 filler is used to move silicon concentration out of the zone of succeptability. Nothing insurmountable, but... Feel really lucky, punk?
Was it Chapman that said "Mild Steel is the sportsman's friend"?