The Alumina line is targeted primarily for vaginal use. It comes from Tantus, who know enough to put flanges on butt toys.
There are a few differences in the materials, some noticeable, some not so much. Visually, aluminum can be anodized, so it's available in colors. Steel is generally made "white" because it's harder to do permanent colors.
Aluminum is a lighter and softer metal. Toys made from it will be lighter, and since it's softer it's easier to machine (one of the reasons, anyway). Aluminum toys will probably be a bit cheaper because of the easier machining (assuming equal pay for the makers). They'll also be easier to hold single-person because they won't be so heavy. The surface of aluminum will require a bit more caution to keep from being gouged or scratched, but basic care will be enough to protect either (put either it in a sock when you're storing it if there's anything hard around, they both will scratch and gouge if you're not careful).
Aluminum is a better conductor of heat (also of electricity, but this won't be as big of a point), but steel has a higher thermal mass. If you insert an aluminum toy and put the outside end in hot water the whole toy will warm up - not so with stainless. Stainless, once warmed up, will stay warm longer than aluminum because of the thermal mass.