Go with the brass if they have a choice, cheaper and sturdier. Copper solders to brass just as well as it does to copper.
Just be sure they don't give you a brass flange for a lead bend.
I have a copper flange out in the shop in case I ever need it, which is unlikely. The biggest problem with an existing house is the danger of using a torch in a confined space. It's easier to cut something off and adapt to plastic.