Alloy wheels are, in general, significantly lighter. Though both steelies and alloys vary somewhat by exact make and model and there may be a little overlap.
That has performance advantages and disadvantages.
- By traditional measures, lighter wheels = better handling and better gas mileage.
- But, specific to winter, a heavier wheel adds unsprung weight. Adding a few pounds of unsprung weight, in terms of traction, may be the equivalent of many times more sprung weight (above the suspension). And it adds to winter traction in the same way that putting sandbags in a light vehicle does.
And built into that is how it's built. Many alloy wheels are designed to be light. And they are stiff and brittle. So in winter's potholes, they tend to crack easier, and any corrosion can accelerate that. And steel, well, they usually make them with plenty of material, and it'll bend well before cracking. That said, yes, they make stronger, more substantial alloys too, they just tend to be on the heavier side of the alloy spectrum, and are often marketed towards off road or rally and things like that. Just looking at how much material is there can tell you quite a bit.
Alloys will discolor but rust is far less of an issue than it is on steel wheels. It's just that it takes a lot more rust to actually harm a steel.