Wow, you're lucky, a free cruise with Princess. You'll enjoy Princess, especially their personal choice dining. I do love that the best over the traditional dining, but you can have that as well on Princess. Which ship are you on? We were on the Golden both times which is a grand class ship. One thing I can tell you is that the balconies are Princess do blow Disney out of the water, they're so much bigger. We didn't even have the largest on the ship as the minisuites are on the Dolphin deck and the larges balconies are on the Caribe deck (regular balcony cabins) and it was huge, a dining table with 2 chairs and then 2 lounge chairs. You may also be surprised at the cabins, no they're not Disney, but they're still nice. We were very worried about our kids inside cabin compared to Disney, but they weren't much smaller, it just didn't have the sofa. I do love the mini-suites on Princess and will have a hard time going back to a balcony cabin, even on DCL.
I went ahead and used my Princess credits towards reserving the 7 nights in March so I now have that as well as the 4 night DCL cruise. I just have to sit my husband down and figure out which he wants to do with the kids. I would ask them, but it was going to be a surprise for Christmas.