Glacier,
Welcome to CruiseCrazies! Your three choices are all good, or should we say great, as there are no bad cruises. We have sailed RCI and Celebrity, but have yet to sail Princess, however, we have heard mainly good things about them. We do believe the food and service on Celebrity is a slight cut above RCI, but that's just our opinion. If the price, timing, and itinerary was attractive would not hesitate to cruise on any one of the three. Also, when it comes to cruising there are no stupid questions.
Congratulations on choosing such a wonderful Christmas present for your SO.