The exact manner and quality with which it is mounted will matter more than the material (provided you aren't using pop can grade material. Steel will add a little more weight, but since you are going to add a red dot sight, a couple ounces (if that) shouldn't matter much.
The key is tolerances. If there is any slop in the mounting, if there is any slop in the fit of the (steel?) rings, over time, recoil will batter the aluminum at the steel to AL joints. Not as fast in a .357 as in a heavier recoiling caliber, but it will happen. Whether or not it will significantly matter is dependent on variables I cannot predict here, but aluminium is softer than steel, so if either one can "get a running start" before meeting the other, the aluminium will come off worst, over time.
Revolvers tend to loosen screws, from recoil. The more recoil, the faster it happens, unless you use a threadlocker. I like "guntite". Screws don't loosen, and they still come out easily with a proper screwdriver.
If you don't use a threadlocker, check them for tightness every once in a while. Screws that are not visibly loose can be loose enough to allow the mount to shift sligtly, which can let it get that "running start" when it meets steel. Can also shift your point of impact.
Either material, of proper alloy, and properly mounted will give a long satisfactory service life. Not properly done will last a while, but how long, no one can say.