I was in the same position about two months ago. I test road the 2006 Specialized Roubaix Elite and the 2006 Trek Pilot 5.2. I wanted to ride the Pilot 5.0, but my LBS did not have any more 06 models in my size, and was going to be another month before the 07 models were shipped. The only thing I really liked better on the Pilot 5.2 compared to the Roubaix Elite was the saddle, and that certainly was not worth the price difference. The Pilot 5.0 compared more closely to the Roubaix Elite, and the Pilot 5.0 did not have the same saddle as the Pilot 5.2.
I do not race, and I weigh 192 pounds, so the few grams savings between the Ultegra and the 105 and the differences in the frame weights did not matter to me. I ended up ordering the 2007 Roubaix Elite. It was received three days later, and I love it. The 2007 Roubaix Elite has 10 speed 105 instead of 9 speed Ultegra rear derailleur. The saddle is also improved compared to the 2006 model. My LBS told me to try the saddle for a few hundred miles, and if I did not like it, they would allow me to trade it in on a different model. I ended up keeping the saddle. I road in the MS 150 two weeks ago, and my butt did not complain.
As Jeff said, you cannot go wrong with either bike.