Depends On Product Nature To Use You May Consider Few Major Points To Chose Aluminum And Stainless Steel Apart From Chemical Composition

1. Weight & Strength : Compare To Aluminum Stainless Steel 3 times Heavier As Per Density Of Metal, Which Help You Out to chose where you want lighter part, this also gives Stainless Steel More Strength Over Aluminum.
2. Life Cycle & Corrosion : With help of Chromium Alloys Stainless Steel is corrosion proof in most of cases where aluminum may corroded rapidly in few conditions and oxidized as well.
3. Cost : Due to weight or density to produce any custom part aluminum is typically cheaper over stainless steel, on other hand as aluminum softer than stainless steel it’s easier to machined and cut which ultimately reduce product overall cost.
4. Other Properties : With few conditions and use of product one can also compare other properties of material like electrical conductivity, thermal conductivity, react to conditions etc.