DW and I really like Vancouver, BC. Enjoy visiting Granville Island, Stanley Park, the Gastown area, Chinatown, ect. Also interesting museums and a great food scene.
Downside is you have to endure the border crossing issues that still can cause traffic backups.
Don't have as much experience with Portland, but it has a lot to offer plus the OR wine country is nearby if you have a car.

How much time do you have and what are your interests? Also, when in the Fall - could be a weather difference.

Seattle and Vancouver are the biggest cities - very urban. Both are beautiful. They have a different feel. Seattle is grungier and funkier with a lot of interesting neighborhoods. Vancouver is cleaner and has an amazing downtown and park system - more planned and organized. Stanley Park and the entire waterfront is great. Vancouver has preserved more of the native American feel (totem pole culture). It is a mix of China and England, in a way.

Portland is smaller and funky - lots of out-of-the box stuff in weird Portland, and gems, like the Japanese and Chinese Gdns.

Not to confuse, but Victoria should be on your short list as an alternative as well. It's a ferry ride from Seattle and is very picturesque and charming. Buchardt Gdns are outstanding.

You may want to throw in some natural wonders, like Mt. Rainier, when you come here, or go to an island. (Victoria is on a BIG island).

So, what do you like to do?

Depending on how many days you have to spend, you could do all three. Fly to PDX and ride the TriMet light rail into the city. the end of the Red line is right outside the baggage claim at PDX. Spend a full day in the city before renting a car for such things as the waterfalls and Mt. Hood east of the city nad Astoria and the coast south of Astoria.
Return the rental car in downtown Portland and take a train to Seattle. Spend a full day in Seattle before renting a car for such things as Mt. Rainier and or/Olympic National Parks.
if you want to see the city of Victoria, take the Port Angeles ferry over. Return to Seattle and return the rental car and take the Amtrak bus or train to Vancouver. You really do not need a rental car in Vancouver since there is a great deal of the city that you can walk or take a HOHO bus to see including Stanley Park. I'm sure you can fly home from Vancouver.
Enjoy your trip to the beautiful Pacific Northwest.

Four days? Twenty-three days? October? November? Do you mind renting cars?

A wee bit more information will really help.

The, uh, Vancouver "in the USofA" isn't the one everybody talks about.

(and you can't even bank on somebody dousing you with acid there - though somebody may douse themselves and perhaps blame it ON you)


If indeed you are willing to allow some of your money outside of the USofA... then Vancouver, Canada tops Portland by a landslide. Vancouver is even slightly nearer to Seattle than is Portland, OR.

There's no right or wrong answer. All 3 are great cities (you need your passport for Vancouver). Victoria is a good suggestion to toss into the mix.