Hi, I had a similar problem when I was awaiting the results of one of my PhD interviews. I had applied for 3, two of which were equaly matched in terms of their desirability (If that word can be used to describe a PhD!!) the other was one recommended by my undergrad uni. I had the three interviews within a week, heard from the poorer one in the same week offering me the post but neither of the two I really fancied. Any hows I simply sent an email to the supervisors explaining that I had been offered a post however, I couldnt make the decision until I heard from them (basically saying I was more interested in what they were offering). They were great about this and got back to me within three days - subsequently I got the better post!! In my limited experience, just being straight up and laying your cards on the table works and supervisors are humans after all (well most of them!!) Hope this helps.