I believe in polymer, that's the best of both worlds, steel is the most durable, but heavy. Aluminum is light but brittle, they crack like frames of guns. Polymer frame with steel guiding rails is the best of both worlds. I would never buy another aluminum frame. I would still buy a steel frame if it's for home defense that I don't carry out.
This is an odd thing to worry about. Current full-size aluminum-frame pistols will last tens of thousands of rounds at a minimum. At today's prices, the cost of the ammo required to break the gun could be well over a hundred times the cost of the gun itself.