It really depends on your mindset. I've always cruised as a party of 2. There is plenty of room in a category 11 or 12 for 2 people. If I were doing that, I would book the 12gty to get the best price. gty means that they can place you in any cabin of that category or above. Since there are very few 12s on the ship, you usually sail in at least an 11 (same cabin, different deck). The size of the cabin is basically the same whether you are in a 12 or a 5--the difference in listed square footage is that the bigger cabins include the measurements of the double bath and the verandah...no real change in living space. You are right...as you move up the scale, you basically get an ocean view and then a "private patio"
An 11 or 12 has a single bathroom rather than the split bath. This isn't a problem for us as I usually work out in the fitness center and shower in their locker room....it might be an issue, expecially for larger families.
Now....for 2 of you for an anniversary.....will you spend most of your time "out and about" on the ship, or being more romantic in your cabin? For romantic time, you can't beat a verandah. Of course, you'll spend major $ more for that verandah room.
Do not book something with the idea of getting an upgrade. Book a cabin where you will be happy. If you get an upgrade, it's a great anniversary present.