Every time of year has its pros and cons, so there really is no 'correct' answer.
Let's see, Late Dec/Jan/early Feb is typically pretty COLD, many rides closed, only one water par (and probably too cold for it) but inexpensive. Mid Feb, prices go up pretty high, as do the odds or warm weather. Typically Feb is pretty risky for cold - I'm talking 30's with wind - no fun for outdoor rides and esp water rides. March and April are best for the weather, but prices are high as are the crowds. Still the hours are long, and it is SO nice to see the lush greenery after a long winter in the north! Easter though is PACKED! President's week also pretty crowded.
In May the crowds moderate, as do the prices, but the temps start to climb. For some reason 85 in September doesn't feel warm, but 85 in May feels icky - esp if ther are crowds. Somtimes there are still crowds in May. Memorial day weekend busymost likely, I've never been then. But we once hit a hot week in early MAy. I didn't like it.
June gets too hot, IMO, and teh crowds build over the month. The rates go up again too.
July - over the fourth is PACKED! but the fireworks are cool. Don't expect to move in MK that night though - many locals come out. AS the month progresses, crowds diminish, but the heat is stifling. prices are still high.
August brings lower prices (after the 8th, I think) but also hurricane seasons. I'm not positive about weather in July, but inaugust, if you don't get full hurricanes - you at least get LOTS of rain. It's rainy season in the TROPICS. Also big thunderstorms. Still, the water rides are great when it's hot...the crowds, not so much.
August into September also brings free dining. If you go then, this is probably the best deal going - except that the restaurants WILL all get booked up. As the kids go back to school, the crowds will diminish (Except those with preschoolers, esp Labor day Weekend) This year, you probably will HAVE to book all your dining at the 90 day mark or else. It used to be 180 days, and they used to not offer free dinign promo until within 180 days. September is still pretty hot and rainy, and hurricaney, but the temps start to diminsh as the month goes on. Also while places to eat are full - the ride lines are not bad. The park hours are very short though. Lots of toddlers as opposed to school aged kids. Many academically serious parents don't want to pull their kids from school in Spetember, nuff said.
October brings better weather (usually), slight price increases, the food and Wine Festival, some locals. A differnet tone is set at night in Epcot during F&W - when the WINE and BEER are emphsized. Lots of Halloween parties at MK in September and October, so no late nights at MK.
November brings cooler waether, the holiday is busy, but eh rest of the month is reasonably priced. Early December is uncrowded. Holiday decor goes up in November now. It is pretty, but not a good choice for a first trip to WDW. - esp. in NOVMBER when it seems odd.
December is fun, but cold. Early inthe month is cheap, late is the most expensive time of year! And the most crowded! (up through New year's.)
Generally, vacation weeks are busiest, most expensive, but have lngest hours to compensate - sort of. Less expensive times of year, have lower crowds, short hours, bad weather. so it's kind of a toss up.
One idea that can be good - is to go during a lesser holiday week. Say Memorial day, Columbus day. You'll get some crowds, and the kids will miss some school - but not a whole week. Like the day of Thanksgiving is brutal - bu tthe week after...the crowds diminish as the week goes on.
Look up Disney's 'pricing seasons.' To ge tmore info. The price difference between high and low seasons is very dramatic! a Standard moderate room on rack rates can go from $149 - $209 plus 12.5% tax each night. The Poly Lagoon view goes from $460 to $705 plus tax per night!
Sometimes though the dicounts offered offset some of the price hikes - SOMETIMES.
Personally, given your two choices, I wouldn't pick either - unless you are from the south. Then it's a major toss up: crowds,heat, high prices vs. hurricanes, rain, heat. If you are thinking this year, I'd strongly suggest October, or early November. Maybe Columbus Day Weekend.
So far this year, crowds at WDW seem to be holding pretty steady compared to the last few years, BTW.
Hope for the best!!
