After switching/buildingh/tweaking bikes for most of the last 6 months, I’ve come to some conclusions about equipment. I’m sure some people will regard some of these as self-evident, but it’s a little different to experience them yourselves after a long period of comparing and testing. I’d be interested to see if others agree:
- Di2 is better than anything else. Etap is not bad at all, and is in many ways more versatile, but nothing matches the crispness and reliability of Di2 (disclaimer – I’ve not tried EPS).
- Disc brakes are better, but not much better - until it rains.
- 105 discs stop just as well as ultegra.
- Weight is not everything, as long as a bike is light enough (for me that’s under 8kg - ymmv).
- Even 33mm deep section wheels make a difference to speed on the flat.
- Even 33m deep section wheels can get a little twitchy descending at speed in crosswinds.
- Nice bar tape is a massively underrated aspect of ergonomics.
- 28mm tires – especially tubeless – transform the comfort of a bike.
- A saddle even slightly too high decimates power, especially uphill.
- Even bikes with similar stack and reach numbers can feel very different.
Conclusions?
My Mason Definition with Di2 is pretty much my perfect all round/year round bike, though I’ll be investing in some winter wheels.
For summer, I’d be tempted with something a little sportier/more aero, but nothing too aggressive. It will have Di2 and probably 40mm deep section wheels - and supacaz bar tape 
