In general, steel wheels will bend whereas an aluminum wheel will break, if subjected to a sudden harsh impact. Based on that, for pure performance applications in a trailer, steel is probably better.
That being said, polished aluminum looks WAY better! (although I just returned from Jackson Center and have to admit that the baby moon hubcaps on the black steel wheels on the new Bambi look sharp).
On a high performance car, you generally want to minimize the unsprung weight. That is, the weight that moves with every bump. So, most high performance cars have aluminum wheels. In the old days they used to make them out of magnesium, hence the term "mags". Only problem is that magnesium is highly flammable and if you overheat the brakes you could light the wheel on fire. They say magnesium fires generate their own oxygen (I guess it's some alloy with oxygen in the chain). I haven't tried it personally, but have heard it claimed that you could light an old VW engine block on fire (they used to be magnesium), throw it in a pond, and it would continue to burn under water. Be pretty cool if it's true
At any rate, on a trailer, the unsprung weight issue is going to be next to nothing. And, the aluminum alloys they use for making trailer wheels are more than likely not 7000 series or something super strong like Boeing would use. So, the wheels wind up being much thicker than a comparable strength steel wheel. End all result is that there's probably not that much weight difference. In fact, I've seen car mags (aluminum actually) that were heavier than normal stamped steel wheels.
Anyway, they're both rated to carry the load so you can pick whichever you like and be OK. If you go after market, just watch the load ratings. I've seen many different aluminum aftermarket mags that are rated at 3900lbs per wheel. I found some online trailer wheel place that had them. Do a google search and you'll find all kinds of them. American Racing makes an Outlaw II that looks like the new ones on a bunch of the Airstream models that is rated at 3400lbs (in a 16" wheel....forget finding anything good in a 15").
Hope that helps,