Cloud Run has a limit of 80 concurrent requests to a copy of your app. Cloud Run has a limit of 2 GiB for memory.
Flex gives you more flexibility over VM shape (CPU, mem), so is suited better to apps that have a higher load. Flex does not scale to zero.
Cloud Run deployments should be faster (Flex provisions a load balancer for each deployment, which can be slow).
Both support deploying directly from container image.
Disclaimer: I work on GCP but have only worked a tiny bit on/with Cloud Run.
