I wouldn't call the 177 improved at all. It's basically a non-aero version of the 168. Might be lighter but it's not an improvement in my eyes. The 268 is an evolution for sure but that weird quick release thru axle thing is a big step backwards to me. But everybody can have their own opinion. I'd still pick the 168 given the choice of all three.

I've ridden the 168, 177 and 268 now. I would agree with the earlier assessment that the 177 has considerable upgrades over the 168 in terms of stiffness and weight. The 268 is a great combination of the latter two and my favorite frame so far. The new speed release style axles took a little getting used to, but have been totally fine and often more convenient.

What direction did you take? 

Tan Tan FM639, still waiting for paint and delivery. IMO Velobuild is less of a gamble.

Price should remain important, although it is trending in the wrong direction...

In the last Nero's show, although they were a bit off on the historical timeline of Chinese frames, they presented what they call a "third wave": branded frames aiming to sell for half or less than traditional brands

Good point :-)
Looks like a good one, maybe the price could be lower, but still..numbers are good and quality as well (I have a 909 allready)

if you're considering the 168, then get the 268. The 168 is outdated in terms of carbon layup. The 268 is reportedly as stiff as the 177, but it's more recent and accepts wider tyres.
Assuming you can fit yourself on it, obviously. If you have very poor flexibility, like if you can't touch your toes, then you probably ought to buy an endurance frame.
As for long term longevity, i wouldn't ride a VB frame from 5y ago because composites tech used has come a very long way (now stiffer, lighter, better finished inside, cable integration, aero, discs, better ride qualities...). in 3y you'll probably want a new frame anyway. Even though modern carbon fibre frames are, in principle, immortal.