Greetings everyone!

I am quite new to cycling and am looking to 'gear up'! My boyfriend is a big cyclist and I've been riding his old bike that's ill fitted and just a sore ride. For Christmas I was given $2500 to purchase a bike. I've been around to some stores, been on a few test rides, been fitted, etc. I think I'm down to either the Trek Madone or the Cannondale Synapse. More specifically the Madone 4.7 or Synapse Carbon 3. So why go with a higher end bike over an entry level... I don't want to feel the desire to upgrade in a year and I have the resources to buy good quality from the start. They're nearly the same price and have nearly the same components. I know most say go with the better fit but I feel like I'm not experienced enough to recognize what the best fit is.

So how do I decide? Are there differences in the carbon quality between the two brands? The Synapse has some Dura Ace(instead of Ultegra on the Trek) and the Trek has a 105 cassette(instead of Ultegra). Seems logical to go to the Synapse because of that but then the Synapse has some proprietary parts meaning difficulty in servicing perhaps..? Is this a reasonable concern? Enough of a concern to go with the Madone that has lesser components?

I'm really torn. Both are good. I just don't know which is best.

Any thoughts?