I agree about the train. Driving across the border at the Peace Arch, which is the easiest when you don't know your way around, can take a few hours in the summer. And the drive between the two cities can be a bit boring, but maybe I've done it too many time
8 days is enough though depending on what you want to do you could spend that much time in either city going to places outside of it. If you are looking to do touristy stuff and just stay in the downtown areas you may not want a rental car in either place. Vancouver has the sky train and while I haven't taken it, I've been looking into it for a trip next month. I believe you can take it from the airport to downtown. I'm going up for a Canucks game and want to stay the night near the arena, but most hotels are more than I want to spend and charge for parking. If you need a car for only part of your stay I would find out if there are ways to rent from downtown so you don't have to pay parking fees and for a car you aren't using.
As far as Seattle, there's plenty to do around town, but we don't have very good public transportation. There's kind of lightrail, you can get it from the airport to downtownish, but I'm not sure I would recommend it. It doesn't go through the best of areas to get there and I don't know if it goes into downtown enough for where you might stay. Like with Vancouver, the places downtown will charge for parking, but I think you'll want a car. You can see a lot of the stuff in Seattle - the waterfront, Pike's Place Market, take the monorail to Seattle Center, in one day, especially in the summer when the sun doesn't go down until around 9pm.
From there, there's Mt Rainier, Snoqualmie Falls, the ballard locks are fun (I think it's the chittenden locks), museum of flight, take a ferry to bainbridge island or somewhere just to take it or get off and explore, drive to West Seattle to see Alki Beach and another view of Seattle (this shows how far away the Space Needle really is from downtown).
It's about a 2 1/2 - 3 hour drive to Vancouver from Seattle, not counting the time it takes to cross the border.