Items is per request, so it's only available for that given user for that given HTTP request. Cache is stored in memory for a persistent period of time, and it not dependent on the specific user. So cache can be shared across multiple users across multiple requests, but Items is per user per request.
It depends on how long you want the data to live. I use Items to store things like the ObjectContext or DbContext in EF, which I only want to be shared for that given request. But reference table data all users will use across many requests, so caching would be better.