My boat is almost five years old and has an aluminum prop on it. I
periodically remove the prop and clean it and lube it and have never
noticed any rust.

Aluminum does not rust. The salt water will pit it and that reduces performance
a bit but if you don't hit anything it should last as long as the rest of your
lower end.
The main reason the people around here like stainless is it will suffer the
slings and arrows of outragously shallow water better than aluminum. SS will
shake off sand, mud, shells and oyster bar hits but a rock will still ruin your
day. Fortunately we don't have much rock here.