I think gratuapairs greatest advantage is it's superior search facility. You will find loads of au pair candidates on there, many of them foreign (hence more interested in leanring to speak English than in the welfare of your kids). But, it is so much easier to troll through the candidate than on nannyjob. But, nannyjob seems to attract more UK based people with childcare experience. It depends on what you are looking for. For example how old your kids are, how many hours of work are involved each week, what experience you want the au pair to have, etc.