Mary,

My sister's best friend died from such a tumour 6 months ago, and at the time of her diagnosis she took the decision to prolong. We thought it unwise, and unfortunately we were proved right, because she only did it out of a sense of panic, trying to hold on for all she was worth, but suffering terribly all the time and in the end was unable to have a peaceful farewell from her loved ones (who stood by all the time, disagreeing with her decision but obeying her wishes).

We talked about it afterwards. Prolonging the inevitable only makes sense in order to put one's affairs in order, whether legal, logistical, emotional, spiritual, whatever. Beyond that I see no reason, so I agree with your preferred option.

Whatever happens, my thoughts are with you and your family, and I wish your father as peaceful and serene an ending as mine, last September (though from different causes).