I have a steel trailer. Wish I could afford an aluminum one.
That said, it does the job quite well and I'm happy with it.
My advice is to decide what you want to spend, look around and see what's available, and determine if you need to adjust what you want to spend.
I think a steel trailer will cost less up front, but more over time, and it will need paint from time to time to prevent rusting. Also, fuel cost of pulling a heavier trailer, over time, will most likely be more.
Aluminum is a great choice if its in your budget, unfortunately, it hasn't fit into mine......yet.