I would prefer using one SSD for OS and apps such as Reaper, Superior Drummer 2.0, etc. for music and Steam for gaming. One SSD for samples and other for games. I would use the HDD for audio projects and other data. Is this the right path?
if you think you won't add more than roughly 70GB of data to the boot drive, then use the 120GB - remember Windows with all it's updates can be roughly 30GB alone, if not more, and remember not to fill it more than 100GB for garbage collection