I do recommend taking the 202/47/Banks-Vernonia trail into Portland. It is very rural and very quiet in terms of traffic and people. Links to maps:
I've never taken 30 the full way from Astoria to Portland, just the portion from Portland-Rainier and Clatskanie-Wesport, so I've missed the worst parts of US 30. But even that little bit makes me prefer the 202/47/Banks-Vernonia routing.
There aren't many services, but there's just enough. Make sure you're stocked when leaving Astoria, as there isn't another full service grocery until Vernonia, about 70 miles down the road. Only a couple of country markets with short hours in between.
The route from Astoria to Hillsboro (Portland westside suburb) is 100 miles. You can break it into two days by camping at Big Eddy County Park which is about 60 miles in. Few, if any, lodging options are found on this route so if you are doing a credit card tour you should be prepared to do the 100 miles in a straight shot. Possible, but there are two small passes to contend with on the route.
I agree with Jamawani that Cape Disappointment on the WA side of the Columbia is a good place to camp. The four-mile Astoria-Megler Bridge is feared by some, but I didn't find the bridge to be that horrible. (However, the Lewis and Clark Bridge at Longview, while shorter, I find worse.) If you don't want to ride across you can always take the city bus: