Hi all,
I'm in a bit of a bind. I am preparing to submit an MA thesis and have fought hard to secure supervision from two relevant professors, who, using bureaucratic reasons etc., had refused to work with me.
After each of them had met with me, suddenly they have become very positive about the thesis and the first supervisor, with whom I had taken a (boring) lecture course, became suddenly very helpful and even offered a PhD position on the spot.
The second supervisor, however, although she is much better—both regarding what interests me in the project and regarding future career prospects—has said I must choose one of the two as my supervisor and drop the other, as they don't get along. Her justification being, somewhat reasonably, that they would pull my thesis in opposite directions and grade harshly what they don't like from the other one. (not really living up to my scientific ideals but whatever)
I didn't even tell the first supervisor about the second supervisor and he didn't ask.

I'm not now sure what do to. I would take the second supervisor over the first any day in terms of purely academic and career reasons. Ethically and for personal reasons, though, I'm not so sure.

In any case, what could I possibly say to the first supervisor to let him down gently and not create a powerful enemy?

Thank you so so much

I am not sure I understand it correctly. Supervisor A offer you a PhD position. Supervisor B offered the same position or a different one? Or Supervisor B is just talking?
You can choose what is good for you. There is no ethical committent to follow a particular one. Nevertheless choosing based solely on academic reputation/research output is not the best idea.