Haines and the ferry to Skagway and you'll see the best of everything. The road out of Skagway is adjacent to the train for part of the trip so you can save the train money if desired.
We didn't see a reason to visit Whitehorse again and took route 8 north of Skagway through Tagish to the Alaskan Highway.
The ferry fare for us was more than fuel cost to drive from Haines to Skagway via Whitehorse. But the ferry is shorter (15 water miles) and very nice.
Be advised that 2 weeks ago a number of places on the Alaskan Highway were closed for the season.