cdn versus in-memory cache for user sessions

hey everyone, i'm wrestling with a decision for a new web app. we're expecting pretty high traffic and need fast access to user session data. my first thought is an in-memory cache like Redis or Memcached, but i'm also considering using a CDN with edge caching for session tokens. what are your experiences with using CDNs for this kind of dynamic, user-specific data? does it introduce too much latency or complexity compared to a centralized cache? curious to hear your thoughts