Well, if you need advice on cruising, you have definitely come to the right place!
I am sure that one of the more experienced members will list some of the differences between the two ships for you soon. They are both fairly new ships and they will both be very very nice indeed.
As far as the questions about dining...the answer will depend on what you prefer. The typical evening includes dinner and a show. If you choose the early dining time, you will generally eat at around 6:00pm and then attend a show that begins around 8:30 or so (that is, if you choose to attend the show). If you choose the late dining time, you will generally attend the show before dinner at around 7:00pm or so. Of course, these are generalizations and the schedule can vary. If you choose one of the fixed dining times, you will definitely be seated at the same table with the same group of people and wait staff every night. With the anytime dining option, I do believe that they can try to seat you with the same wait staff, if you request that, otherwise, I think it can vary. In my experience, early to bed, early to rise type of folks are better suited to the early dining time and night owls do well with the later dining. For example, if you sleep in and eat breakfast at 10:30am, then lunch at about 3:00pm, you really won't feel like eating dinner at 6:00pm. Of course, you do not have to eat dinner in the dining room (although, I think that is my favorite part of every day) and there will be many other options if you miss your dining time.
As far as hurricanes go...there could possible be a delay in your departure/arrival, change in itinerary, or the cruise could be cancelled altogether, depending on the storm. It is just too unpredictable to know what will happen. The ships do travel relatively fast and carry advanced weather equipment on board, therefore, they are very good at avoiding really bad weather.
As far as other advice...the number one mistake that I think most cruisers make is that they over pack. Think about what you will be doing everyday (hanging out on the sun deck by the pool, or in the casino, or whatever else) and plan accordingly. Funny story...my wife mentioned after one of our previous seven day cruises that she never once wore a bra! She was either wearing a swimsuit top under her clothes for excursions or hanging out on the ship, or wearing nice dresses in the evening that did not require a bra. Those are the kinds of things that you may not think about. Also, Carnival ships offer laundry service with about a 24 hour turn around for a fee. Some of the ships also have a washer/dryer area that you can pay $3.00 per wash and $3.00 per dryer if you get desperate. Typically on a 7 day cruise, there are two formal nights (formal attire is suggested but not required) with the other five days being casual in the evenings. Check out the Carnival dress code for more information about what is or is not allowed in the dining room.
Make sure that you have the appropriate proof of citizenship now. Do not wait until the week before your trip. Although the US government does allow the use of birth certificates in some instances, passports are highly recommended. If you go to your local post office or passport office now, you will receive them in plenty of time and they will be good for ten years for anyone 18 or over.
Other than that, keep checking/searching these forums for a wealth of information about cruising and have a great time!