Security Assist will be all you need to create a registration system with login, forgot pw, log out etc... It would not include validated forms, user administrative back end, or reporting charts to view trends.
If you purchase the user registration solution you will have a pre-canned site that can do all that and more already created, but you won't have all of the tools used to create it so you would be editing by hand unless you also bought all the tools, not just security assist that were used to create it.