100 amp sub panel

Hey guys, I'm doing some work in my basement this summer. 70-year-old house and we tore out the old creepy cinder block camp style bathroom, and we're going to put in a nice jetted tub with a heated floor. My father-in-law is going to help and he used to do contract work about 30 years ago. We need to run a 100 amp subpanel from the garage to the basement, roughly 70 to 80 ft. He really wants to do copper, everything I see says aluminum is fine. Copper equivalent to aluminum 2 2 2 4 ser is almost five times the cost.

What do you pro's use? Can you point me to relevant code sections to point out that aluminum is fine to use? Copper cable alone if we do that will be more than my jetted tub.