If you are prepared to do a little traveling as crisis occur with your parents, I'd definitely follow the grandchild if your son is agreeable.
Grandparenting is a whole new phase in your life, a wonderful, satisfying one. I truly believe that my father's life was extended a decade by the birth of his grandchildren and his wonderful relationships with them. Or maybe there's a middle ground temporarily that will grant you a little of both. It seems that you are anticipating this birth with more joy than you've felt in a long time. Embrace that bliss!!!!