Hiya Michelle!
These are also two of our favorite times to go!
My kids were all about Disney World during the Holidays until we experienced Halloween for the first time last year and we were all absolutely gobsmacked! I think it was the outrageously fun Mickey's Not So Scary Halloween Party that did it...that was so much more that I expected, full of Disney magic.
Your kids are close to the ages of my kids (mine are 7 and 10) and they are at some an awesome age where they really know what they like and this is prime memory making age.... in other words, they will remember the vacations you take now for the rest of their lives!
I think it will make the kids feel like the very important family members that they are if you tell them that they can choose this year...and maybe alternate it the next time you go to Disney.
We are shooting for Halloween this year and my kids having been planning out costumes all summer long! Ok, I'll admit it...so have I!
Whichever time you choose to go, I hope that you have a fabulous time, full of the magical memories that Disney helps us with!