Diffuser Question

I have decided to go down the pie pan route (9 inch) for my mini build, any thoughts on the type of metal? I have looked at Aluminum and Stainless Steel, there is not much difference in the cost and would lean towards the SS, but wonder about the heat transfer.

I'll take stainless over aluminum any day of the week and twice on Sunday. Shouldn't mess with temp too much unless it has water in it.