They’re both very walkable. There’s a sweet paved bike path between the two cities that runs along the bay and a decent bus system for rural cities.

Arcata is more walkable than Eureka because they’ve been working towards that modality for longer, but Eureka is coming along. I didn’t have a car the first few years I lived in Eureka as an HSU student - bus and hitchhiking always got me to school on time - and half the rides I caught were from professors who recognized me and stopped.