There's no "easy" answer to your question, but let me comment on what I think is the easiest from 25 years experience of Alaska cruising. I've found May (especially) and June to be the best months weather-wise to cruise Alaska. The weather there is very changeable, but I've found those two months to be the best.
As for Seattle versus Vancouver - lots of variables there. Usually, air fares are less expensive to Seattle. I, personally, love Vancouver and prefer cruising out of Vancouver, but my personal preferences shouldn't influence someone else.
You need to compare the itineraries of cruises out of the two cities in question and decide which is more interesting for you. Victoria is usually a port of call on Seattle departures (sometimes only 4-5 hours)
but it's not always a port of call on Vancouver departures. However, it's easily reached from Vancouver for an all-day or overnight tour.
Hope I've given you some food for thought?