Accepting and then declining PhD offer,
As you as you are professional and upfront, this shouldn't cause any hard feelings. Go with the one you like the most, after all, it is your research and time that you will be spending. You can always change your mind and email professor at A with a brief message explaining that you prefer to go to a different school. Keep in mind that you will be just starting your PhD and who knows if you will have the same opinion about going for a post doc or maybe get lucky and find a faculty job! Opportunities can rise out of no where. 

One more question, why did the other professors suggested that you go to A and not B or even their university? Do they know the professor at A? Or because of the university's reputation or lab?