For any computer today, server or no, I’d probably default to SSD today unless I expected to be making use of a large store of files that I expected to access in serial, like a large movie collection or maybe a backup server that can play well with rotational drives.
The only thing there that looks like it could be doing that is the Samba server, depending upon what the remote clients are doing with it (could be a movie server).
In general, if you can fit your stuff on an SSD today, I’d get an SSD.
You also can also add a rotational drive down the line if you run low on space and need inexpensive space for something that you’re going to access in serial, and use both; just move the bulk stuff to the rotational drive then.