Installed software probably won't take up a lot of your space. 10GB is more than enough for most people.
However, the config files and other data can take up a fair amount, so...
IMO, install mint on the SSD, but use the HDD for your /home partition.
Depending on your usage, you may be able to get away with putting everything on the SSD and just use the HDD for downloads, music, videos, ISO's, etc.