It can depend. I'm coming from Cubase, so I'm a bit new to S1. Although I'm not new to SSDs... at all. I have 2.5TB of SSD storage on 3 computers totaling 128GB of ram, networked using VEP connected to S1. For large orchestral templates (film scoring) I have each instrument family (woodwinds, brass, strings, percussion, etc.) on their own SSDs for the highest speeds. Kontakt, VI-Pro, and any decent sampler will take advantage of your SSDs speed. So that's a win. But I also built the newest machine with 2 SSDs in raid for the drive I put Windows on as well as where my song files will be. I plan to install a program that will automatically back up the song files to a storage drive, to cover my bases. But the point is, I moved projects to the faster drive because it always annoyed me a bit to have auto-saves and things take so long (although that was Cubase, and I find S1 to perform at least 10 times faster at everything. ;) Still, I like the new layout more.
That sounds like it's a bit different than your setup. But my point is that SSDs are fast. So the real question is... what do you want to see improve in speed? I've put care over time into making sure the right things went to SSD. But in the end I put all of them on SSDs. I only use hard disks for long term storage now. So... what's the important thing for you? Well, how much RAM do you have and can you upgrade to on your machine? Will putting Superior drummer on an SSD allow you to load less into the ram? If so, then do it. If you use other instruments that will better use the SSD, then use those to free up RAM and let Superior Drummer use the RAM instead. That's why I say it all boils down to your setup, your needs, and machine.
How many of which instruments, using which software vs how much of RAM vs SSD you have. It's like a 4 way graph, depending on what your projects look like.
Hopefully I didn't make that more confusing. :)