tell them how difficult the decision is for you, and you need to think about it. Emphesize that they are the best bosses you could ever have, and emphacise that your decision is not based on their performance, but rather based on your vision of the your future career. You tell them what you want (for example, furture career, your phd, cutting edge tech), and you let them tell you what they can offer. If they keep dancing around, without solid offer (for example 10% increase with title, and everything, forget it). Anyway, just keep saying how thankful you are for the opportunity of working with them, how much you have learned, and how difficult the decision is.But do let them know that you have more aspirations for your career, and what you want in the long term.
When do you need to decide for the offer you have? You could let the other side know that you have a counter offer, this way, you could leverage, and if they really want you , they might be able to make a gesture of increase your salary a little bit (probably not too big). You should play this game.
