It comes down to a couple of determining factors.
If you need the data loaded into RAM swiftly, you will want it on the SSD. If the item is read occasionally, or only at start-up (example Excel document when opened loads to RAM and works in RAM until saved. It does create backups in the OS drive, though) then the platter drive is best.
If you are a gamer, you could benefit from storing your most RAM intensive games on the SSD, as well. If you are a creative, things like Adobe Scratch Disk files would be best on the SSD.
SSD: OS, Apps, Games, Scratch Files (Virtual memory files) for some apps
HDD: Long-term storage, items that are small, backups, archives, etc.