SSD - OS and applications
HDD - Large, for all that other stuff.
Having the OS and applications live on the SSD significantly speeds things up. Even more so if you can put some of your files on that as well.
For instance, a mediumly complex Excel file
On my office PC, approx 5 secs to open. All HDD
On my home PC, approc 0.5 sec to open. Mostly SSD.
The exact same file.
The SSD speeds things up that need to be. Applications, etc.
The HDD for large drive space.
Slowly, my main system is becoming SSD only. It currently has 5 drives. 4 x SSD and 1 x 3TB HDD.
That HDD may be moving to a different system here soon.
My Adobe Lightroom application and catalogs live on the SSD. Weekly, when you close it, it does a backup of the catalog. Takes maybe 2-3 secs. If that were to live on the HDD, it would take much longer.
3 secs vs 10 secs does not seem like a lot, except when you see that delay over, and over, and over.
Once you use an SSD, you won't go back.