That is a very difficult question and I think it comes down to how much load the alloy part is carrying.
Aluminum has a finite fatigue life. You can make the aluminum part thicker, exactly like a steel part, and the amount of loadings it carries will increase, like steel, but unlike steel, where you can make the part so thick that it will have an infinite fatigue life, you can’t do that with aluminum. Which is why an aluminum aircraft has to be totally rebuilt after XXX flight hours. Steel is too heavy for an aircraft so airlines have to live within the limitations of aluminum.
Certain designs, such as revolvers, the frame carries a significant load and in time an alloy revolver will crack. It may take 10,000, 30,000 rounds, but it will crack. Other designs, such as an autopistol, I am certain that the heavy load paths are carried by steel. So then lifetime becomes an issue of wear. I do know the Walther P38, originally a steel autopistol, when the post war frames were made out of aluminum, something bad happened because later P1’s have a steel reinforcing pin in the frame.
As others have said, the amount of ammunition you will have to shoot will be more than the cost of the pistol, so, buy what feels good in the hand.