When the concept of cost comes up, it is such a personal decision that it is tough to offer commentary. $600 is to some an entire month's salary, to others it is what they spend on groceries each month.
What I would say since you haven't been on a cruise before, is that most people do not spend a lot of time in their cabin. There are plenty of common areas on the boat, both outdoors (which may be chilly), or inside with plenty of portholes, windows or floor to ceiling glass.
We found that about the only time we spent in our room was sleeping, changing clothes, or resting between activities. Otherwise, we were watching movies or shows at the theaters, visiting characters, swimming in the pools, etc.
You can get plenty of memories for $600 (professional photo CD from Shutters, nice dinners at Palo, shore adventures, etc) I would think this would be a better use of funds to help take a little bit of Alaska with you. I would hate to not have any money for shore excursions in Alaska - I hear that some of these are fabulous for whale watching, etc.
Good luck, just being on a
Disney cruise is fabulous. There is not a wrong choice here, just personal preference.