I'm real new at this - but my thoughts for picking AL are the ones you mentioned.
A big concern I gather, is excess weight in the gantry (if moving gantry mill).
Plus - I don't weld - would have to get it done - I'd want the unit square - so would need a welder I'd trust (and make sure they had a frame/chasiss jig to keep it that way).
I also like the "tinker-toy aspect" of aluminium frames!
Jim