I have a friend who has his Class III SOT (or at least that is what I think he has...

) over in Missouri. We talked a little bit about suppressors over lunch one time. He said stainless was the way to go for suppressors since cleaning would be a cinch by dipping it in a container of 50/50 vinegar and hydrogen peroxide.
I am here:
http://www.onlinemetals.com/calculator.cfmfiguring out metal weights, for an 8.4" long by 1.25" outside diameter SOLID piece of stainless tips the scale at 2.96 pounds.
a piece of aluminum of the same dimensions and again SOLID weighs 1.01" pounds.
So right off the bat, it looks to me that aluminum is about one third the weight of stainless.
Like that guy at the cnc link above, I would probably go with the first blast chamber/baffle out of stainless too. I had a friend who had made his own compensators for USPSA open division pistols. He tried to make one out of aluminum, but with the gas forces of 9mm Major it actually would deform the first baffle inside the comp. So he stuck with stainless steel. He tried titanium once, but I think machining it is a whole 'nother skill level and tool level above hobbyist/homeowner grade.
I guess if you go to make it out of aluminum just plan ahead or be prepared to have to take the thing apart to be able to clean it or scrub each individual part by hand.
I don't have a lathe...so if I lived in one of those free'er states where I could make one, I think for my first suppressor, I would just try making one out of freeze plugs.