Where Focus groups would be more credible as there is an essence of personal interaction involved. But this also depends on the researcher and how involved he/she is in the focus group. This allows the focus group to communicate their opinions beliefs rather than just a collation of multiple choice questions which don't really explain fully how they feel about the issue in question. Although the credibility of Focus Groups depends on the how the observer analyses the discussions and how the focus group is arranged in the correct way to tackle the hypotheses in question.
References:
Focus Group, Retrieved 17 November 2010 From:
http://en.wikipedia.org/wiki/Focus_group
Morgan D, L. (1997) Focus Groups as qualitative research 2nd Edition. Retrieved 17 November 2010 From:
http://books.google.co.uk/books?hl=en&lr=&id=iBJZusd1GocC&oi=fnd&pg=PP7&dq=focus+groups&ots=mwSfaMeBX2&sig=r5PU3loC0G-opR5aUKr_6BUPMTc#v=onepage&q&f=false