Trek 5900 vs Colnago C40

No comparison. The C40HP wins by a mile.
And if you don't believe me, when there have been questions put to large numbers of pros about what bike they would most like to be riding, the C40 wins by a long way.

True story:
In February 2001, I was in Majorca, Spain at Stephen Roche's bike camp. The Banesto team was staying there, training and preparing for the Majorca Challenge. I had my 2001 Trek 5200 OCLV. In the mornings, the campers and the Banestos left from the same garage where all our bikes were kept over night. The Banesto pros loved to come over and ask me about the 5200, pick it up and just look at it--which amazed me because their Pinarello's looked so sweet. One pro spoke pretty good english, he told me he always wanted to ride a Trek. We started talking about bikes and I said I liked Pinarellos and Colnagos. He said he rode the Colnago carbon the year before when he was on a different team, he shrugged, said it was ok, but it did not handle that well on descents and he thought the Trek was a better bike. Go figure...I bought a Trek 5900 in 2002, I like it even better than my 5200.