If you install an SSD it'll give you faster boot times and applications / games loading times. If you install bigger RAM, it'll give you much smoother experience in resource hungry applications like games. These days 8 GB is a kind of minimum for a gaming PC, so you'll see a significantly smooth experience with games.
So it's up to you to choose. If you are buying as SSD larger than your current used capacity of your HDD(C Drive), you can simply clone it using a migrating tool. Else you'll need to do the migration manually.