I have a Seven Elium SG, which I love. I weighed (*cough*) 188 pounds this morning (seasonal). The Elium has Ti down tube, head tube, and chainstays while the top tube, seat tube, seatstays and fork are carbon. In the Elium's case, this supposedly gives a stiff drivetrain (Ti) with a more comfortable ride (carbon).

But, my belief is that materials aren't the only, and may not even be the major, factor. Bike design and how well it's constructed can easily trump materials used.

Here's a quick article that might be worth a read:
http://www.bikecyclingreviews.com/Ti...arbon_Mix.html