The short answer is 'both' ... or 'neither'.
The fact is, every situation is individual, and no two are the same - the only common factors that unite them are the loss of a loved one, and the guilt and worry left for those who grieve.
I think in the circumstances, you made the best of an impossible situation, given that you could not be with both people. I think your dad would feel you were in the best place, and i am sorry you did not have chance to say goodbye to him, for your sake, not his, as it sounds like he would not have known if you were there or not.
Please don't hurt yourself any more over this. You had to make a decision, and live with it, and you did the best for someone else, not yourself, and no-one could ask for more than that.
It's not good for you to wonder 'what if' when you can do nothing about it now. You did what was right at the time, and in this imperfect world, that is a circumstance that was thrust upon you, and you dealt with it with love and selflessness. That is what you should use to comfort yourself.
As for the throwaway comment of a thoughtless and unfeeling nurse - that does not bear remembering - so forget it.