Howdy Haley! Let's first discuss
Disney Vacation Club. If you are remotely interested in joining DVC, speak with a Guide so you can learn all of the specifics regarding DVC Membership. Think about your vacation habits and determine if DVC Membership is a good fit for you and your family. When you speak with a Guide, there will not be a high-pressured sales pitch. They will simply explain the benefits and pricing of a DVC Membership so that you can make the most informed decision for your family. (Note...we have been Members since 1997 and have treasured each and every moment of our Membership!)
If DVC Membership is not the best fit at this time, you might consider AAA Vacations.
(AAA.com/DisneyworldResorts) You will receive a fabulous package, a great discount and they will add some extra magic to your vacation.
Regarding July vs. December. If December, what week are you considering? Christmas time will have extremely heavy crowds. My personal preference would be December but with school schedules, that can be difficult for families. December is truly magical. Not only do you have the typical Disney magic but you have the addition of the holiday decorations and events. The weather is typically nice though we were there one year and it was quite chilly. We are from Texas though so chilly is relative. July is also great! Depending on when you are there, it could be hot and crowded but you will be able to enjoy longer park hours and will definitely want to take advantage of the pools at your resort.
I don't know if I have helped you with your questions but I do know that whatever you decide, you will enjoy each magical moment!