I've taken both routes recently and it's a hard decision to make. Cassiar is great for it's remoteness and fantastic BC provincial parks. The drive into Stewart / Hyder is breathtaking as you weave along a glacial valley. Not so great is all the damage done to the highway by heavy construction trucks putting in that eyesore of a HV power line project... what a scar on some truly beautiful and remote land.
Yellowhead from Jasper to Kitwanga is more scenic in my opinion than 40/97 between Jasper and Dawson Creek (or Fort Nelson for that matter.)
AK Highway does have some gems on it, but also many many MANY boring miles, it gets better the farther North / West you travel, but there's a lot of miles to cover before "the good stuff".