If you have the money get all SSDs.
But what Jim R wrote makes most sense.
At this point, I figger the only reason for SSDs is to reduce noise.
Disk drives are a very mature tech. Reliable, and cheap.
Once the computer boots the OS disk is seldom accessed on an audio optimized box.
So what! the computer boots a few seconds faster. Optomize and save the money for a bigger disk. Always can use the storage.
3 disks on my comp:
500 giga audio files
1 tera OS, and storage: backup for audio files
3 tera samples, storage: tutorials, downloaded installation files, and pictures of a few million naked women (my preference and comfortable with it)