It really depends on what you intend for the rifle. So far, I have always used White Lightning vents made by Jim Chambers. I could make my own easily, but at $5-6 it really isn't worth my time. There are other options out there too... Beryllium coming to mind first. I think Beryllium is a popular choice in Europe these days. This somewhat surprises me, since Beryllium is quite poisonous in dust form. I know this because my father installed one of the first MRI's in my state, and he was issued beryllium wrenches because they are non-magnetic. I used to help him a lot, so used these wrenches myself. Beryllium was used because of it's toughness, and resistance to wear, which is exactly what you want in a touch-hole liner.
I don't have a personal muzzleloader. I have an SKS, but not a personal muzzleloading rifle (One of these days, I hope to build one for myself.). But the ones I build for sale are SS linered. I believe it will give my customers the best, longest service. I used to hunt with a caplock that I borrowed on an almost daily basis, and I know that a rifle I make may also receive that kind of service. I want to make things that last as long as possible.
Whatever liner you choose, please do the gunsmith a favor and use anti-seize compound on the threads. Same goes for the breech plug as well. This will keep things from rusting into a solid chunk of metal.
M. E. Pering