There is no Asahikawa Onsen. Do you mean Asahidake Onsen? I am going to assume so.is it enough time to see anything
Yes, at either resort, you can take the ropeway up, walk around a little and take the ropeway down again. Sounkyo also has some nice waterfalls a short taxi ride away. Asahidake has some walking trails. If you arrive around noon, you have enough time to enjoy the area a bit.
which one of the resorts is the best one to stay in and to see best scenery?
Both resorts come with noce scenery. Asahidake Onsen is considerably less developed than Sounkyo. Sounkyo is located in a steep, narrow valley, while Asahidake is surrounded by rolling, forested mountains.
More details:
http://www.japan-guide.com/e/e6775.html
If we can stay for 1 night extra would this be recommended?
Unless you want to make a major hike or relax some more, more time is not really necessary.