Up to you, but a good mild steel brand will be aluminized and won't rust anyway. The Manta ones are all aluminized, the one that I know about that lasted the longest was 5 years before the guy sold the car and after that I don't know, but it was still good when he sold it.
The Manta 2 1/2" stainless is $1795 shipped. Not the cheapest, but as I said, their mild steel systems still last for years.