Hike from Sounkyo to Asahidake or vice versa?

We're planning a 5-day trip to Hokkaido in our Japan Holiday end of September 2018.
One of the things we would like to do is the hike from Sounkyo (ropeway) to Asahidake (ropeway). Starting in Sounkyo because then you have to climb less then starting in Asahidake. However I read most people start in Asahidake.
Can anyone explain why most people start in Asahidake? (Descending is easier for us then climbing).
How much time it would take to travel from Asahidake back to Sounkyo?

We're planning a 5-day trip to Hokkaido in our Japan Holiday end of September 2018.

Note that there is a chance of snow before the end of September. It may not be possible anymore to pass the trails without special winter hiking equipment and skills.

Starting in Sounkyo because then you have to climb less then starting in Asahidake. However I read most people start in Asahidake.

It is probably because many people stay overnight at the Kurodake hut before descending to Sounkyo on day 2. Apart from that, it does not seem to make a big difference. I think the bigger issue is the context of your trip.

How much time it would take to travel from Asahidake back to Sounkyo?

About 8-9 hours including the ropeway rides. It is a long hike. Only well prepared people in good shape should attempt it in a single day.

Or if you ask how to return to your departure point after the hike: by public transportation it takes about half a day. You have to travel via Asahikawa.