If you're doing one or the other, skip Portland/Seattle and do California.
Ir you're interested in the pinnacle of current American brewing, you want to do anything in your power to visit Port Brewing/Lost Abbey, Russian River, and Stone, and to a lesser degree Alesmith, Ballast Point, and the Bruery. Green Flash, Bear Republic, Lagunitas, and the like are incredibly awesome also-rans.
If you're interested in the history and mass craft brewing, you want to see Anchor and Sierra.
All of those are in CA.
Seriously, Russian River, Lost Abbey/Port, and Stone are 3 of the premier breweries in the world.
I've been to many of these and can vouch, but to add a bit of perspective
Alesmith, Ballast Point, Stone, Green Flash, Port Brewing, Lost Abbey are all within 30 minutes of each other. The Bruery is about 2 hours north in Orange County, and is right next to Bootleggers which is lesser known but also awesome. Lagunitas, Anchor, Russian River are all in the Bay Area which is about 6-8 hours north of San Diego driving. Between Orange County and the Bay the only place I could think of worth stopping is Firestone in Paso Robles.
I know sometimes people think that California is California, but as an example I've never visited the Bay Area stuff because it's kind of a big trip for me to get up there. Stupid tall state
Hypothetical Journey Of Awesome West Coast Beer:
Start in San Diego. Spend 2 days there visiting all the amazing. Brewery tours in the day, awesome local bar scene at night. At the end of Day 2, drive up north to hit The Bruery and Bootleggers in Orange County (which are night time tasting rooms). Stop for the night and maybe visit Disneyland or something
Day 3, start heading north early and stop in Paso Robles to visit Firestone. Then press on up to the bay area. Visit local beer bars that are potentially awesome. Stay the night
Day 4, visit Anchor, 21st and try to make it over to Lagunitas. Stop again for the night
Day 5, head up the coast, hit Bear Republic and Russian River. If bold, continue on towards Fort Bragg and hit North Coast and maybe even farther up to Eureka to hit Lost Coast. Somewhere in here you should probably stop and move something to the next day.
Day 6, power on to Portland. Check out a bunch of the Portland stuff that people here have recommended better than me (Rogue I'd hit for sure, one of my faves)
Day 7, more Portland. Possibly take one of the different beer tours I've seen there
Day 8, Head up to Seattle and cruise around to some of the sights there
Alright, this is indulgent for me because I'm planning on making a trip like this one day when I can. But driving up the west coast you'd get a huge variety of beer, as well as locales and people. Big cultural differences between San Diego, LA, the Bay, Northern Cali, and the Pacific Northwest. It'd be a hell of a ride