IMO, stainless steel cages are superior as you can bend them to adjust tension and they're hard to break. Also, stainless wont make dark marks on your bottle like Al alloy. Plastic or carbon cages can be fine and look better on modern bikes but some designs tend to be bottle launchers. Alloy cages can be great (Elite Ciussi Gel is awesome for gravel) but mark up your bottles and can sometimes snap if you bend them repeatedly. Ti cages are unnecessary from a practical perspective. All weigh about the same.