The aluminum is is lighter in weight, and has ION -X strength glass display. The Stainless steel offers a more durable sapphire glass display. Where the Stainless steel is susceptible to tiny scratches over time the aluminum tends to have deeper scratches. In short the stainless steel is more durable than the aluminum not to say the aluminum isn't still a durable option. With its lighter weight the aluminum watch. is more ideal if your looking for more of a sports watch where, the stainless steels gloss finish offers a more professional or classic nice watch design.