I've a machine with two HDD's and a 40 GB SSD that I wish to install Ubuntu Studio on. Which setup makes more sense?
OS installed on SSD. Media on HDD.
Media on SSD. OS on HDD.
The OS should always be on your fastest drive. SSD or not. That is a golden rule on a workstation. While that may well be my subjective opinion, I think most people will confirm this after having tried running with couple of high end drives in raid 0 for their OS/Applications partition, or even a single mid-class SSD drive. The reason is simple: everything loads faster.
Having said that, it may be worth noting that creating a temporary ram-mapped filesystem for temporary files will be desirable in some cases, while in other cases, putting the temp files on the fastest drive will be sufficient.
Once your OS and audio applications are loaded, the interaction with the hard disk should be fairly minimal, just logs and user setting updates and the like.
And then you would be loading, editing and saving audio files, plus creating temporary files possibly.
Putting these on the faster media would seem sensible, and it is worth understanding if your audio applications use temporary storage, and have them point that at the ssd also.