I would say the same but it depend on what's at stake for the career. If its something I can overlook or work it out another time, I would go for the family function. Family is always first that's why we do what we do to give them the best even if we are not going to be with them.
For an understanding family. They would understand that it is what it is when it comes to career. But Christmas holiday with family can never be missed. Also I know a secret on how to make you come to my wedding when I'm ready for it (Bestman)
The question already said that your career is at stake, so it's not a matter of understanding what's at stake. Family is important, yes, but a good family will never have to make you choose between them and your source of livelihood. There are many Christmas to be had, it's not as if you will always be behind schedule at every Christmas...
Hahahaha I will leave your invitation hanging so you won't be able to invite me.