Any recommendations or ideas on whether to do the Western or Eastern cruise?
Both are enjoyable.
Eastern
On St. Maarten the beaches are very nice (you can do on your own if wanted), the snorkeling can be iffy because they get a lot of wind which kicks up the sand and reduces visability. The Golden Eagle Catamaran excursion always looks like fun (a bit pricey but sailing, swimming/beach and snorkeling in one package). There are some "nude" beaches on the French side of the island, but it's not a big deal really - we took a 8, 11 and 14 yo there and they are not raving sex fiends now. Orient Bay is a beautiful beach (clothing optional) and there are sections of the beach for families, topless and "the full monty", Just ask your cab to drop you off at the family beach.
St. Thomas has a lot of nice beaches and St. John's is great for snorkelinig (many excursions here). Trunk bay is a great place to snorkel - it's a national park with markers telling you what you are looking at.
Western
Key West - if you like "partying" it's the place to be. We took a snorkeling excursion and it was very good. Grand Cayman has the "not to be missed" excursion - Sting Ray City (we missed it and went on a snorkeling excursion instead). Our excursion was some of the best snorkeling we have ever had. We were in front of where the Magic was anchored. That was way cool. Cozumel = Carlos and Charlies for the partying crowd. Lots of very good snokeling here or just enjoy one of the many beaches. THere is also a fun sounding jeep trip through the "jungle" to a beach - it sounds like fun.
Which cruise to take is the first of many very difficult decisions you will have to make!
2) Any advice on what type of room and/or ship level to request?
For a family of 4 I would get at least a cat 10 room (split bath-sounds odd is GREAT). Remember, you are really buying the service / food / shows / entertainment. It is the same for ALL guests, so you might as well buy the cheapest room on the best ship - and you have picked the best ship - the Magic!!

The cat 10 SPH rooms are a very good deal (only 6 on the ship). If you think that you need some outside light, the cat 9 rooms have a very large port hole. We started in a cat 11, cat 10, cat 10 sph, and cat 9. We didn't miss the outside light until we had an outside room (didn't know any better

). Now we sail cat 9's. Never had a veranda, so we don't miss it.
Second hard choice
How dressed up are the fanciest dinners? Do I need to bring a suit or would a sports coat and tie work? What about for my wife??
See the thread for the debate over formal wear. I'm a shirt and tie guy for formal and semi formal. I brought a suit once, wore the coat to the dinning room, took it off, ate dinner and carried it back to the room - so for me - never again!!
To "formal" or not to "formal", that is the question!
What excursions are a "must do" for either cruise?
See answer to number one. For us snorkeling is the "must do" excursion - just deciding upon which on is the problem. If we go to St. Maarten again, I'm for trying the Golden Eagle and if we go to Grand Cayman, it's Sting Ray city for us (probably using someone other than
DCL - I've liked the reviews of Nativeway)
What are the peak seasons/months for the cruises?
Maybe what you are asking, is when is it less crowded and the best deals. Obviously, hurricane season (Sept through Nov) is a prime time for hurricanes (the ship will divert around them-don't worry). January and February are also "off season". We took our kids out of school during MLK week in January and had the best weather of all of our cruises. August is hot, Hot, HOT (and humid - and did I say
HOT! )
Any ways to save on booking the cruise?
BOOK EARLY and off season. Sometimes
Dreams Unlimited and other TA's will have room credits and group discounts (even though you are not a group - if they get enough bookings on a sailing they will give a discount)
anything else we should watch out for in planning
Get the video from DCL, if you get the Travel Channel, they have some cruise shows: general, 4 or 5 different ships featured and some that are a full hour on a ship (yes there is one about DCL). The web sites that others have mentioned: CastawayClub.com, DCLTribute.com, wdwig.com/cruise.htm
Have fun. Ask questions. Don't be afraid of controversy - we all have our opinions and still like each other. Luv Ya, Mean It.
Once on the ship things not to miss:
The main shows in the Walt Disney Theater - Disney Dreams is not to be missed.
The family cabarette shows between dinner and the main WDT shows are a lot of fun (in Rockin' Bar D and Studio Sea)
Relaxing in a lounger (hardwood with blue padding) on Deck 4
Adults: Art of Entertaining series
Palo Brunch
Ice Carving demonstration on deck 9
Croissants (freshly made), the plane ones and the chocolate filled ones. White Chocolate covered Domes (and or Creame Brulee') for dessert in Lumiers.
Send a post card to home from each port you stopped in
Watching the ship dock at least once from deck 10 forward - so you can look down on the captian as he parallel parks a 967 foot long ship (if only I could park my car as easily

)