Looks like you are planning on taking the ferry to either Haines or Skagway and then riding from there, right? If so, the route from Haines over Chilkat Pass will be the best overall, as it has more miles of scenic wilderness before you get to the Alaska Highway.
The ride from Skagway up over White Pass and on to Carcross is spectacular, but once past Carcross it becomes rather blah by comparison. And the stretch from Whitehorse to Haines Jct is nothing to get terribly excited about either. Whereas from Haines you start out right away running along the Chilkat River then, at the border, you start climbing up 40 Mile Hill into the pass. There is little in the way of civilization until you reach Haines Jct, and before long you're back out in the wilderness, on your way to Kluane Lake.
But... you can get it all if you really want to. By getting off at Skagway you could take a quick ride up to the top of White Pass - probably make it up to U. S. Customs and back down in time to catch the ferry back over to Haines. The time the ferry stays in Skagway varies, depending on which one. The Columbia, for instance, is scheduled to be there for 3 hours, while the Malaspina (which makes round trips between Juneau and Skagway) is there for just 1 1/2 hours. The reason I suggested turning around at U. S. Customs (just before customs, actually) is so you don't take the risk of being held up getting back down to catch the ferry, although you could also schedule it so you stay overnight in Skagway and then return to Haines the next day on a different ferry.