9001 is the best mechanical groupset Shimano has made to date. Front and rear mech action is the best ever. The crankset and rings are super stiff.

The brakes are bang-on the most precise, allowing great predictability. Add to that the direct mount option which just rocks (if your frame has that).
Lever ergonomics are excellent as well. All the components are a step up, and I loved 7800.

Compared to 7800, there is nothing that is better than 9001. The only argument is if you prefer the smaller ergonomics of the 7800 levers. 7800 sti levers were better than 7900 levers but that's about it.
Overall, new Ultegra is better than 7800.

It's kind of a moot point since you can't buy 7800 new and people are not going to cross-shop 10s used to 11s new.
If someone were looking for the best shifting period, they would get Di2.
Mechanical supremacy belongs to 9001.

I wonder if Shimano will intro a new mech DA groupset or just focus on Di2. I think they are due this year for all-new product.

11s vs 10s- 3 options: 1. a higher top gear 2. lower bottom gear 3.more tightly spaced ratios

11-28/pro-compact or compact 11s pretty much covers most sportive riders and some racers
tied to a standard crank, you get that extra gear for climbing. That's what I notice most on 11s.