I would test ride as many different brands as you can. Felt and Trek are both good, so it really comes down to feel. For me, Felt's geometry wasn't quite right; the top tube was too short and that was that.
Other bikes to look at might include:
Scott Contessa, Fuji Finest, Specialized Dolce (all aluminum, generally with carbon forks), and Trek Madone, Specialized Ruby, etc. (all carbon).
As far as aluminum vs. carbon, they have their pro's and con's.
Aluminum is more reasonably priced. That's probably its biggest asset. On the down side it tends to transmit more vibration up to the rider, so it's less smooth.
Carbon's more expensive, and a little more fragile. A crash can crack it fairly easily. But it's lightweight, and dampens the vibration making for a smoother ride.
Ride both, and see how you feel about them, relative to your budget.
2011 Specialized Ruby Comp / Specialized Romin 143
2009 Trek 7.2FX WSD