I favor the coastal route, with strings of campsites up the rivers. This would have allowed those living on the arctic steppe desert to access both the herds at choke points, aquatic resources, and the greater vegetation present near the rivers.
The people living near the river mouths would have had usable boats, even if I doubt that they could or would willingly travel more than 3 days by boat before camping on land. This is due to the ability to store potable water and average time between storms.
Hides are used to store water and they would have had pottery also from their trades with the peoples living in what is now the sea of japan. Southern coastal towns would have been permanent, not camps and they would have been primarily fishermen of salmon which would have been available during most of the year (run timing). The Jomon in Japan have a documented culture going back 14,000 years.