i hate summertime in orlando. too hot. so my oct UY is great since i would never travel to wdw during july-august-september.
but for someone who mostly goes to wdw during july-august for their summer break from school, an oct UY would be more risky if they needed to cancel.
(and there is also the consideration of whether you want a more common UY month in case you intend to add-on later - a higher supply of resale contracts with your UY might make it easier.)
in any event, keep in mind that UY has nothing at all to do with when you can book your resort. that is based on your home resort only.