Trek Domane 2.3 vs Domane 4.0

[cosmicbike]

Thinking about an N+1 under the C2W scheme in a month or so. My choices are limited to Trek, Estway and Felt.
Current road bike is a basic Trek 1.2, 2013 model with Sora triple groupset.

I was looking for a bit of an upgrade, and swinging towards one of the Domane series for a more comfortable long ride machine. The reviews seem favourable from a comfort perspective, so I'm just wondering what the benefit will be in going for the 4.0 over the 2.3?
Will I notice the difference in having the carbon frame? And Tiagra vs 105, given I'm pretty happy with the Sora?

 

[StuAff]

The 2.3 gets pretty good reviews, but I'd suggest stumping up for the 4.0. Better to have a higher-spec frame than higher-spec components. Road.cc (review is the 2013, not the lighter 2014 version) reckoned the carbon was considerably more compliant. Cycling Weekly were a touch unfair on it, IMHO, but also said 'go carbon'.