Get the largest SSD you can afford. After 2 years of having a 120GB SSD I am about at the point of upgrading to a 256GB model just to have more space. I hate having to uninstall games to install new ones. Some games can be 20GB by themselves.
Also keep in mind you need to keep at least 20% of the drive free for TRIM and garbage collection to work correctly. Since a 120GB drive formats to 111GB that is 22GB you need to keep free.