In practical terms? None.
Stainless is ussually the cheapest, easiest to make, and heaviest (Aftermarket stainless exhaust is still way lighter than stock). Titanium is the next lightest, with CF being the lightest of all. The thing to watch out with CF is heat, as it melts. While riding the exhaust will cool itself alright, but if you run it hard while standing still, like on a dyno or doing burnouts, you will damage it. With either metal you dont have that problem.