Alloy = cooler.
If you hit something hard enough to bend an alloy rim, it will likely bend a steel rim too. Maybe not as much, but once either is bent, even a little, they're toast.
Also, some steel rims have a flat spot in them at the weld when new. Quality control.
I would always replace a steel with an alloy, unless you're trying to keep it original.
Besides, alloy is cooler. Oh, already said that.