Baking Powders can be fast-acting, slow-acting, or double-acting, depending on the acid or acids they contain. Some acids react with liquid, while others react with heat. Double-acting powders usually contain two acids—one for liquid, the other for heat.
Aluminum free Baking Powders react with liquid and not with heat. Which makes them faster acting than most double-acting powders. You need to move fast and get cakes made with aluminum-free Baking Powders into the oven promptly since most of the bubbles are released shortly after mixing."
Baking Powders with aluminum, on the other hand, have most of their action delayed until the batter is hot in the oven.
So aluminum free Baking Powders are better than Baking Powders with aluminum.