In spring, the Alaska Highway usually is the more reliable route.
Spring conditions can be variable on the Cassiar, including closures due to flooding. Later in the summer, the Cassiar is in better shape as the road work gets complete.
Biggest problem in the spring are the potholes. The freeze/thaw cycle is hard on the asphalt then the traffic breaks it up, creating potholes, some huge holes now and then. The Cassiar north of Dease Lake is subject to these potholes, making it a slow go through some spots in the spring. Roads crews get out once or twice a month to fill the potholes but you have to keep your eyes open until then.
You're not staying in Alaska for the longest day/shortest night of the year (June 21)? Always a fun experience with many communities having celebrations. Dawson City, Yukon has a good party to celebrate, likely to be an even better one this year with it falling on a Saturday. :B