Aluminum is more pliable than stainless steel, and aluminum is also the lighter of the two. Aluminum works better for applications requiring parts with straight deep walls or needing shapes that would push stainless steel to the cracking point, according to Wenzel Metal Spinning.
Steel also features a higher density than aluminum - almost 2.5 times as dense. This means that steel has greater strength and is not as likely to warp or bend when placed under significant heat, weight or other force. Aluminum also does not rust and does not need a coating to protect it from corrosion, while steel needs treatment to keep corrosion at bay, as stated by Wenzel Metal Spinning.