I think heading north on the Alaska Highway and south on the Cassiar Highway gives the best experience.
Several reasons for my thinking.
The Cassiar is usually in better condition later on in the summer, after the repairs are done from the winter abuse.
The Cassiar is narrower and rougher on the north end, so it improves as you head south, which is nice.
On the Cassiar, the bears are more active later in the summer and early fall. If you want to see bears, the Cassiar is one of the best routes to drive in the fall.
Since we've done the Alaska Highway many times, we prefer taking the Cassiar both ways now. The Alaska Highway is busy with gas/oil workers & trucks. That's actually another reason to do the AK Hwy northbound...in the late summer and early fall the AK Hwy is extremely busy with rig workers/trucks/big loads south of Fort Nelson. In early summer things aren't quite so hectic as they still have much of the season ahead of them.