As an assistant, you usually get six years (in contrary to four years for a PhD) during which you divide your time between the preparation of your PhD (about 70 % of your time) and rendering of services towards the faculty, which usually consists of a teaching assignment.
As a PhD candidate, you can always apply for vacancies for assistants, so check our vacancies page regularly. It does not necessarily mean that you have to have excellent degrees in order to be considered for a job as an assistant: everything depends on the quality of your co-applicants. Of course, you have to be extremely motivated to carry out research and to teach in your faculty.
As an assistant, it will take you longer to obtain your PhD degree in comparison with a PhD student, but on the other hand you get the opportunity to acquire further skills in several areas: research, teaching, rendering services, … . This kind of diversification always comes in handy when you enter the labour market after obtaining your PhD.