While the Cassiar is our favourite route we will take the Alaska Highway if there is any concern regarding the weather or conditions. The Alaska Highway is well-travelled, has more services, and is maintained better (quicker response during a snow storm).
If you take the Cassiar, half of your driving will be on Hwy 16 from the junction of Hwys 37 & 16 to Prince George. Hwy 16 is easy driving (almost boring), well-travelled and maintained well. There are more services along this route than the Alaska Highway between Dawson Creek and Watson Lake.
Hwy 97 between Dawson Creek and Prince George goes through a high mountain pass which can get snow early.
Alaska Highway has the Liard Hotsprings which are very nice in late September.
I think it is six of one and half dozen of the other ... check the weather forecast and road conditions (at the info centre in Watson Lake) before going either direction from Watson Lake.