Christina:
Although I have not stayed at either of these hotels, I am a Hilton Honors member and have stayed in many Hilton hotels. Although it's hard to beat the cookies at the Doubletree along with its larger suites, the Hilton may have something besides location that might draw you there: Disney Magic Hours.
Guests of Hilton can go early and stay late and utilize the Magic Hours priviledge that is usually only reserved for guests on property. This advantage may not seem valuable, except that you are visiting during a very busy time of the year. However, with Magic Hours, you could potentially avoid any disappointments that crowds can bring if you use the early and late park entry.
Before you make your final decision, I would ask Double Tree if guests have the Magic Hours priviledge (I couldn't find it on their website). I also recommend taking a look at reviews on a travel site like
www.tripadvisor.com. I don't know that being one more mile down the road will make too much more difference, if you're staying off property, but the Magic Hours might move me.
Wishing y'all a magical vacation!
Whitney