I have a 2017 Domane SL6 disc and it is significantly different from the 2014.
For the reasons poorinrichfield cites in his post, I really thought about trading up to the 2020 but can't really justify it by need. The 34 tooth rear (vs. my 32) and that downtube storage are the only real adds I would get and I can upgrade the drivetrain to get that - and I don't really mind the saddle bag.
If I had a 2014, I probably could have convinced myself!