Asahidake Onsen vs Sounkyo Onsen

I wonder if I have time to visit only one, which is better ?
Japan-guide gave Asahidake 1 more red dot but in all the guidebooks, they favoured Sounkyo.
Hope to hear some comments from those who have visited both. What's their major differences ? If I can extend my stay to visit both instead of 1, will one added day be enough ?

The main difference is that Asahidake Onsen is much less developed than Sounkyo Onsen. So, those tourists who prefer a less developed town are likely to prefer Asahidake over Sounkyo. But those who prefer to stay in a town and at larger ryokan may prefer Sounkyo.

Personally, I also find the scenery at the top of Asahidake Onsen to be more interesting than the scenery on top of the Kurodake Onsen.

If I can extend my stay to visit both instead of 1, will one added day be enough ?

A large detour is needed to travel between the two towns. The trip takes under two hour when done by your own car, but it can take considerably longer if done by public transportation. So, if you travel by car, I think a single additional day can be enough.