Spiffs vs Eeprom on esp8266
On the ESP8266 there's very little to choose from between the two. It all really boils down to what you want to store and how you best want to access it.

EEPROM emulation is ideal for small bits of data - configuration values and that sort of thing. Where you just want to store a few of values that would seldom change.

SPIFFS though is better at dealing with large amounts of data. Since it's a filesystem it can store actual files, and that makes it ideal for serving images and static data for web pages, for example. 

A SPIFFS filesystem can be built on your PC from files of your choice and then uploaded over serial using mkspiffs and esptool.py or the SPIFFS tool that can be installed into the Arduino IDE. That means the files are very easy to update.

SPIFFS is not so great when it comes to writing data though - experiments have shown that it gets quite slow when writing often.  EEPROM doesn't suffer from such a problem, but then you can't write as much data to EEPROM as you can to SPIFFS.

So that's really all there is to it. Neither is "better" than the other, but each is better suited to different ways of working or to different styles of data.