When I had a custom bike built, I went for steel rather than titanium. I've never regretted my choice, but I have always been curious. $1K more would have got me titanium, which in retrospect was a pretty good deal. Maybe I should have done it. The frame I just bought is nearly the same size, so it will give me an idea if there is any difference. I definitely feel a difference between each of 4 steel bikes I have, so if I can somehow separate expectation bias ...
I think if I were to spend any more on a frame, I would have a custom one built.