Well I have to admit, I've never tried either. But I have had other parts made of both. So, FWIW...
- Titanium will have a small flex, but then stop. Not good in pedals. Ask me how I know
In a saddle, I'm thinking it should be good.
- Manganese I tried in a Pinarello frame once. Very stiff ride. The bike was solid and didn't seem to flex at all. I never tried it long distance, so I can't say if it was a harsh ride. I didn't end up buying it for different reasons. I don't know how it would fare in a saddle.
I'm also a clyde at 260#. I use a Selle Italia Turbomatic saddle with carbon fiber rails. I have over 8,000 miles on it and have had no problems whatsoever. I had tried a Fizik Arione saddle in the past with steel rails. I broke the saddle shell itself 2 years in a row, warranty'd them, and finally broke the rails. Then I went to the Selle Italia.