New SSD. There are a few ways to do it.
1. I always recommend just a fresh install on the new SSD
2. Reduce the total space on your current C drive to 80% of the actual space of the SSD, and migrate
3. There is one application that can probably migrate only
the OS. http://www.paragon-software.com/technologies/components/migrate-OS-to-SSD/
It is not free.
4. See #1.
Good choice on the SSD.