Blades that are made from stainless steel are the strongest, and they can withstand almost anything. If you are using the blender almost every day, especially to grind something hard or crush ice, you are going to need blades that are extremely durable. Stainless steel blades can also tolerate heat and strain; so, you can keep on using it without worrying about the blades deteriorating.
Also, for safety purpose, make sure the blades are inside the jugs and not on the motor! Otherwise, mistakenly turning on the motor without putting on the jug can lead to a serious injury.