New Rider: 2011 Trek Madone 4.5 vs 2012 Cannondale CAAD 10 3

[Teena]

I have never ridden a Trek, but I have three Cannondales and I adore them all. I have 2010 CAAD 9 Feminine 5 and I love it almost as much as I love my husband (he thinks maybe even more!) It is my favorite bike. I have ridden thousands of miles on it and it is awesome. I have heard people say aluminum bikes are more "buzzy" because they are stiffer and you feel more of the road bumps. But the stiffness make riding hills easier, I feel more powerful and the bike practically rides itself. It is incredibly responsive and I feel like I am one with the bike when I ride. Last year I did three centuries and many rides of 60-plus miles and never felt like the road vibration was intolerable.
I also have a Cannondale Synapse and it's not quite as awesome, probably because it doesn't fit me as well (and I did have a pro bike fit). It's my winter bike. My third C'dale is a 20 year old M500 mountain bike.
Another thing I consider (and this is just the tree-hugger in me) is that once a carbon fiber bike is done (broken, cracked, whatever happens to cause its ultimate demise) it can't be recycled, and it ends up in a landfill where it never goes away. Aluminum is an element that can either be recycled, or remains inert in landfills.
I also agree with the sentiment about the more masculine color being a more re-sellable feature. If the CAAD you are looking at is the black one with the orange and red girly-swirlies, I think that bike looks SO cool!!!
Having said all that, I'm SO close to getting a custom titanium bike. I'll probably sell the Synapse at that point.
Good luck with your bike search.