They are totally different vacations. We did the 7 day cruise once on the Magic, and although it was really cool, I won't do it again. This was in the spring, and we had 30 foot swells for two days. It was an experience that is for sure! And you will have extra costs, tips, some drinks, etc... We spent an extra $1000 onboard. Plus the packing is different. On the cruise you need clothes for dinner as well as casual stuff. Some folks get very dressed up also. My kids were 8 and 10, and my 8 year old was barely into the kids clubs, my 10 year old (at the time) really wasn't into it at all. The nice thing is they could come and go as they wanted around the ship, so they enjoyed the freedom. Younger kids would love the activities, though! My friends' younger daughter was in the clubs from 8am to midnight pretty much daily. She loved it.
It really is a different vacation than going to WDW. It is so busy! There is always something going on. At WDW you can relax more I think, and go at your own pace.
The reason I would do Disney this year over a cruise personally is the whole flu scare. My son got sick on the cruise - severe bronchitis due to his asthma, high fever, on steroids, antibiotics, and breathing treatments (now I carry a portable nebulizer anywhere we go). He's the type that will get sick easily. I would be more comfortable on land this year. Don't want to scare you or make it seem like I'm paranoid, but that is just me. I did like how as you entered the dining rooms, they handed antibac wipes to everyone. Also the kids had to clean their hands every time they went into the kids clubs.
Disney cruise is really cautious about cleanliness.
Funny thing, though - my son (now 12) has been asking when we can go on another cruise! I may take the kids with a friend or my mom some time again - my DH didn't like the cruise thing much. He likes to relax by the pool at the resort more - the pool onboard is small and busy. We would do a different cruise line, though, as my kids are not into the characters, and others cost less.