Personally, our decision was based on timing as well, knew it had to be during spring break-up as the FI works for the Oil & Gas industry. Second thing was the itinerary and then availability for the wedding and times that I wanted.
Since sailing with Princess and wanting to be married "At Sea".... also being quite picky, had to be the last wedding of the day and the first day at sea. After picking which itinerary and then which ship came down from 2 Itineraries, 2 Ships, 8 Sailing Dates and found the one that worked best for us and that was the final decision of booking our room and then the package.
Enjoy picking out your date and itinerary, I had lots of fun picking out ours!