To keep up with the SO's race bike riding and on her special request, I needed something to ride fast.
Now (edited Dec 2017), this has been superseded by a carbon road bike modification where I modified a Shimano Ultegra brake and shifter lever set for use [link]. If you are after the fastest, coolest, lightest, then better go there maybe.
Also, a different prosthetic approach is used there: a VC body powered prehensor.
Here, a fixed adapter with permament position for the whole bike ride has been available at the time I performed this choice of materials and modifications.
The case for flat handle bars
There are somewhat uncomfortable iliosacral joint issues that many bikers around my age have. There is the issue of having to move the shifters and levers to the left side on the handlebar. So the BMC Alpenchallenge with its conventional sitting position and a horizontal handle bar seemed like the obvious choice. Rather than a race or road bike setup.
The shop sales person the advised me to get the Shimano SORA version. So this made my choice a BMC Alpenchallenge AC01 SORA. Not the belt variation, which was definitely the right choice.
The BMC Alpenchallenge AC01 SORA comes with a 3550 50/34 chainwheels and a HG50 11-32 cassette. Thus, a 50/11 gear ratio for the Shimano SORA contrasts with a 50/19 as maximal gear ratio for the belt drive. So the conventional and not very expensive SORA transmission clearly wins the speed question.
Now, BMC introduced the Alpenchallenger line specifically for road bicycle riders that wanted the conventional "Herrenvelo" format. This is definitely a real bicycle (not some motorized gadget). It wanted to go just as fast as we rode our good old Swiss Allegro "Herrenvelos" back in the days: fast as hell. In other words, the Alpenchallenger was introduced as a commuter bike that can serve to chase e-bikes.