DH and I tend to go most years in May and October and find October is just ideal. The weather is perfect - usually warm enough for shorts, swimming, fun in the sun but not too hot, and evenings are nice and pleasant. We have gone always with discounts - either AP discounts, or basic code discounts. The discounts are usually best if I'm not mistaken around this time or at least quite good. We have also taken advantage of the great rates at S/D, but I'm not sure if they're guaranteed to be low every October. I have never been to the Halloween party, so can't comment on that, but October is ideal for WDW...somewhat lighter crowds, especially from summer and don't forget the wine and food festival.
Good luck and have a great time!