Don't hesitate to book a 12. Most of the time, you will be upgraded to an 11 (which is the same room on a different deck).
You didn't say how many people you will have. The only thing you miss out on in an 11 or 12 is the split bath. If there are 4 of you, that can be an issue. However, I found that I tended to work out in the morning and shower in the locker room...which allowed the shower in the cabin to be free. Also, my teen and I didn't tend to get up at the same time.
We have been on 12 cruises on the Magic. The first time, I was in a category 6--the verandah was nice....since then, I've booked the cheapest available (usually an 11 or 12). Twice, we were upgraded. However, reality is that the difference between an 8 or 9 and an 11 or 12 is the porthole and the split bath. The difference in the square footage of the cabin is the split bath.
We tend to spend very little time in the cabin, so the lack of a verandah or window wasn't a big deal to me. DD definitely likes the window, but as I've explained to her, she can have a couple days at Disney World or she can have a window on the ship--suddenly she doesn't miss the window. If you are into a private breakfast on the verandah....or if you have a little one who needs to nap and you want to hang on the verandah while they sleep, that is nice. Again, for us it isn't worth the $. I'd rather cruise twice in an 11 or 12 than once in a 6.
It is the same cruise--same service (wonderful), shows, food, etc. whether you are in a 4 or a 12. The only differences come in when you choose concierge level (1-3). That MIGHT happen for someone's high school graduation, but not for "routine."