Hi Bob,
I agree with your ideas, Aluminium window doesn't mean bad. Especially in your living condition.
"Thermal Break Aluminium Window"with high quality maybe one of your reasonable option. Dual-glazed(with or without air/argon)??Functional durable effective??
Wish it could offer any help to you.

Hi Bob. I just went to the Milgard showroom and was floored to see they have an aluminum line. It's new and he says it's the most expensive they have - more than wood! He says people like it for the look, but fiberglass is the most durable. Aluminum has more issues with expansion/contraction with hot/cold temps.

BTW, if you are at high altitude you probably don't want argon. In fact, Marvin's warranty EXCLUDES "glass installed more than 5000 feet above sea level without capillary tubes." Capillary tubes would leak all the argon out so you don't want to paying for argon. I live at 7000 ft. No argon for me.

Argon is denser than air and will stay in the IGU if handled properly.

You will not get the same fill rates but you will still have some benefit of it.