I've always found something of interest at the Seattle boat show for each of the 21 years I've lived here. Never been to the Vancouver show, but it is a very fun city to visit. You probably won't be disappointed by either. I realize that's no help, sorry.
Have attended both for the last several years. The Seattle show is much larger. That being said the Vancouver show, is a good one. Also you get a day in a great city. We usully do both, this year our plans are to attend the Seattle show.