Tough Call - The south side has The Historic Columbia River Trail (Rails to Trails) and sections of the Historic Columbia River Hwy (open to traffic) It also has some stretches where you'll be forced to ride on Interstate I84. I hear the ride along the Rowena Crest is real scenic too. I've never done it - not sure how tough route finding would be. I know there'll be more elevation gain. The Washington side is narrow and sometimes busy. IMO the Oregon side is more scenic. There's access to many hiking trails to waterfalls. The Oregon side of the Columbia has a listed 77 waterfalls.

If you're okay with lots of elevation gain and noise when riding on Hwy I84, I'd go with the Oregon side. I've driven both sides, but never ridden them. A web search might turn up more information. I'm not sure how tolerable riding on I84 would be so I'm not advocating either side.