In response....
1) least expensive is usually hurricane season - from Sept ~ October + first week of Dec and some of January. I like labor day in Sept cuz you get a holiday but low rates and cheap air to boot. BUT it is the peak of hurricane season too.
2) FOr the carib - I think it is just fine in Sept so long as YOU don't get disrupted by a hurricane - most ships will sail around a hurricane or reroute the ports for safety and to maximize entertainmnet value. HOWEVER - NCL is being sued in a class action for sailing a ship THROUGH a storm front to make port - their passengers were uncomfortable among other things.
DCL last year did was a "class act" for how they handled matters with hurricane disruptions.
We were scheduled to sail in early Sept - and Hurricane frances cut our 7 day to 4... and the embarkation port was shifted to Fort Lauderdale. Disney refunded half our fare + discounted 25% off a future booking + provided a free nite at their Polynesian resort + WDW Ultra passes + free Coach transfers to Fort Lauderdale and back from Port Canaveral. We would have driven a rental car to port canaveral if we didn't have the hurricane disruption. My point - DCL cut the days - refunded $, discounted future sailing and simply bent over backwards with a comprehensive crisis support package - compared to NCL who said "full speed ahead" through a storm front to make port - that was DUMB of NCL.
On the other hand - I did sail NCL in Alaska in 2003 and it was a very nice experience. Nice ship - and "free-style" dining - plus very nice alternative dining venues for a fee. AND they discount children's fares scaled by age - and dirt cheap - so very family friendly on the pocket book - just like Disney. Not many mass market cruise lines treat kids like this!
3) The Wonder and the Magic are twin sister ships. I have always sailed on the 7 day Magic schedules so I cannot speak from personal experience here.
4) We have always sailed just the cruise - never a split land sea package. Reason so is because we're from Southern California and we have have annual passes for
Disneyland. However, we are considering buying the 7 or more day WDW pass with unlimited duration - this way our daily cost is held to about $25 or less and we can use it for a few days each year until it's used up.
COnclusion: the experience is quite different from a land only vacation - AND it is quite different from any other cruise line (in general).
We've sailed Carnival (4x); NCL (1x); Holland America (1x); Rennisance (1x); RCCL (4x); Disney (2x); and the Yangzee river 1x ... plus a handful of other minor and now defunct small ships lines. The one significant difference from DCL to all the others which is NOT obvious is that most of the other lines had staff which were less motivated and at the worst - the staff were slipping in small comments or remarks to fish for better gratuities. But NOT by doing extra and more attentive things - rather they were directly asking or talking about gratuities or being over-worked - or similar.
Exceptions were Holland America and about half the ships on Carnival and RCCL. The WORST cases were from Ren ~ but then again REN is now defunct - or rather it has been reborn as another LUX line with prices about DOUBLE what they used to be. We have yet to sail Princess; Celebrity or some of the pricey lines like Crystal, Seabourne or Raddison. Raddison was going to be our summer deal in the MED this year - but the family gathering thing fell through at the last minute.
SO the staff - all the way from the cabin steward to the deck servers + the purser staff ALL come across seamlessly to make for a superior experience on DCL. PLUS with the Disney entertainment environment it all comes together to trump overseas trips and competing cruise lines alike. I imagine as our kids grow older and more sophisticated we'll return to other curise lines and land trips which emphasize culture, the enviornment and history more. PLUS, frankly I can't wait til both kids are 6 so we can do a family WINDJAMMER trip - as these hold interest for me as "off the big ship beaten path" trips.
I have found a wealth of info on these boards - both DIS and the likes of Cruise Critic (but I prefer DIS for Disney stuff obviously). I would advise you use the "search" feature to zero in on things of special interest - but even so you can find simply tons of minutae with potential use in the current strings of the forums.
A FINAL remark on HURRICANE season. We have sailed about half a dozen times in September. Only ONCE did we have our schedule disrupted by a hurricane. Frances cut our 7 day to 4 last year and Disney bent over backwards for us with a diverse range of supportive gestures. YES - sailng in September IS a hurricane season - and if you're careful to check with NOAA (as I have learned recently) they put our forecasts for each season.
LAST year AND this year have been described as extremely unusual years with muchhigher then normal activity. Maybe in 2006 it will begin to calm down back to normal. Of course - most of us book our trips LONG before the NOAA announce the forecasts. HECK! What we've learned is If we have a hurricane threatening the schedule - so long as we can make the ship safely we are going to sail - and DCL is now established as standing by with comprehansive "fixes" to make the shortened trip enjoyable.
Good luck with your plans!