This is funny, because I went thru the same dilemma - and made spreadsheets. Was considering the Magic 5 day very merrytime out of miami 12/10 and the Fantasy 7 day very merrytime 12/2 out of PC. I showed the cost of gas, hotel and mileage (all more for us if we were to sail out of Miami), and still the other cruise was quite more expensive. So, I could not sell the 7 day purely on cost per day. (the difference in price was about $1200, just for your reference)
In the end, 2 things convinced my husband to go with the 7 day, were A. not having to drive to Miami - the hassle, the hotel, all of it. We can be in PC in 1.5 to 2 hours and no need for a hotel. B. my obsessing over planning and his worry that i was going to regret not doing the 7 day (this happened on our first cruise on the magic). There just wasn't enough time on the 5 day, i felt we missed stuff, and i immediately began planning a new cruise when we returned. So, he said, if you're gonna regret it, lets just do the 7 day so i don't have to hear about it. For me, it had to be Very Merrytime, as I haven't experienced it, and I'm a bit obsessed with Disney and Christmas. (we also are not doing Disney during December this year, as a compromise, and it was a selling a point to say that trip money would be applied to the cruise, and Christmas will be a lot cheaper as this cruise is basically the kids' gift).
I also work very hard on the money saving aspect of this, which goes a long way. Disney vacation account, gift cards, etc... I always lead with how much extra money i've managed to make or save, and then hit with the cost. Also, I have to agree with the poster who mentioned womanly wiles...that tends to help alot...