Categories 5 and 6 are exactly the same, except for the deck they are on. The rooms themselves and verandahs are identical.
I personally like the category 7's, because they're ususally about $300 cheaper than the 6's, $600 less than the category 5's. However, some don't like the obstructed view as much as they do the standard verandahs, which have a plexiglass railing that you can see through while sitting down. Since I like being on deck 7, I've gone with the category 7's for most of our cruises, since being in a 5 on our first trip.
There are 4 cabins which are now sold as category 7, but were initially cat. 6 or 5. They have slight obstructions, but nowhere near what the regular category 7's have. The room #'s are 7120, 7620, 6134, and 6634. If you book one of these, you're essentially getting a 5 or a 6 for the price of a 7. We're in 7120 for our next two cruises. Here's a
link to a site which has some pictures of these.
I hope this made sense, there are a lot of 5's, 6's, and 7's in there....