It depends on the average size of your files. If they are large parts/assemblies or renderings, I would think the RAM will be better. If they are fairly simple the SSD might help more but I have some reservations about saving files on the SSD - reliability.
Monitor your memory usage with the Windows Task Manager (Ctrk-Shift-Escape) and the Resource Monitor. If you are using all of it, then new RAM will help a lot. If you are using most of your 8 GB, then new RAM is probably going to help. If not, then it won't help at all.
I am going to assume that 8 gigs of ram is enough and you are not running out of it when you work.
An SSD will make the system a bit peppier since all the reads and writes to disk will be faster.
Either upgrade will not help any CPU bound performance issues.
Cheers,
Anna