That depends Samira on a number of factors - have a look here for the pros and cons of both methods http://www.s-cool.co.uk/a-level/psychology/research-methods/revise-it/questionnaires-and-interviews 

Can I suggest you do both - start with a mail out - a survey and then select a group to follow up for qualitative interviews - then you capture numbers and meaning and interpretation. Good luck with your research.