Agent is a simple solution that could work for smaller loads and a few client processes. ETS table should usually perform better, and can support concurrent clients, i.e. you could have simultaneous multiple readers/writers - something not possible with Agent/GenServer. It is however very limited in terms of atomic operations, so it’s mostly suitable for simple k-v stuff, and some concurrent counters.
Personally, if I know that there will be multiple clients of a key-value store, I just go for ETS immediately, because I believe this is what it was made for. That being said, some cases are in the grey area, so starting with a simple Agent is a somewhat simpler and more flexible solution. Assuming you encapsulate cache operations with some module, switching to ETS should be easy, because you’ll likely need to change the implementation in only one module (the cache wrapper).
Finally, as others have pointed out, think carefully whether you even need a cache. All other things being equal, cacheless is better than cacheful (because of less complexity), so if you can get away without it, it will be the simplest solution 
