After talking to numerous people on this subject in the past and from my own experience, here's my 2 cents worth.
Race car - run em both in titanium
Street car - titanium intakes, SS exhaust
Another thing, and people may argue with this, but I think titanium valves are only good for about a season and at that point they need to be replaced. This point has been debated somewhat but I had a titanium valve disintegrate on me after being in use for about a year, replaced them and no troubles since.