The biggest difference between aluminum and stainless steel baking sheets is how they heat up and how they retain that heat. Aluminum will heat and cool more quickly, whereas stainless steel takes longer to warm up, but it also retains the heat for much longer. Most bakers will prefer one over the other for personal reasons or depending on what they bake the most.
Aluminum is a very light, thin metal that will heat up very quickly. Aluminum baking sheets are usually uncoated. You may find your baked good burning or sticking to the sheet. Therefore, you will almost always want to spray your pan with a cooking spray or otherwise lightly coat it in oil or flour before putting your goodies on the sheet.
If you’re baking on aluminum baking sheets, you may have to adjust the temperature of your oven down slightly, or possibly take your food out of the oven a few minutes early. The first time you bake a certain recipe on your aluminum cookie sheet, start checking 5-10 minutes before the timer goes off to prevent burning.
Aluminum sheet pans must be hand washed - dishwasher detergents are too alkaline and therefore corrosive for the aluminum and can produce white spots or blacken the pan, and in some cases cause pitting and corrosion.
Note: Aluminum may also react with certain foods, especially acidic food. This can change the color of the metal or the food your cooking, and leave a slightly metallic taste to your dish.
Stainless steel baking sheets are usually treated with a non-stick coating, which may eliminate the need for greasing your baking sheet before using it. Stainless steel is a heavier metal than aluminum, which makes it more durable and likely to last longer. It also means it takes longer to warm up.
If you’re not used to this, and if a recipe was developed using an aluminum pan, your baked goods may not be thoroughly cooked when the timer goes off. One way to compensate for the longer heating time is to place your pan in the oven as it warms up. By the time your oven is pre-heated, your pan will be too. You can then take it out of the oven, place your unbaked goods on the sheet, and return it to the oven. Stainless steel also retains its heat for longer than aluminum.
Stainless steel is non-reactive. It will not pick up any unwanted, unpleasant or harmful flavors.
Stainless steel pans can be put in the dishwasher.