@RWB, I’ll be testing some Particle.function() code to command the photon to fetch a specified file into flash. Other commands could be used for display sequences, listing files, etc. I’m just bantering ideas about.
The main difference between the SDFat and SPIFFS is that SDFat is optimized for SD and has been around a long time. The author has also created a version for Particle which uses SPI DMA for maximized speed. I don’t know enough about SPIFFS but it seems to depend on the user to optimize operation via callback functions. The biggest difference with an SD card is the mechanical aspect as it is designed to be removable whereas chip flash is not. The environmental conditions (shock, dust, heat, accessibility,etc.) should dictate which is best for the application. As for power, both provide low power operation though again, the application will dictate the best solution. There is no “one shoe fits all” solution here.