I would say YOUR decision should be based on your experience, comfort with stretch goals and drive. Your decision would have been a lot easier a few years back; before half marathons came into vogue. So what you need to do is look at training plans, the distances involved as well as time required to commit to those distances. I would then look internal to see if you will be satisfied with running the half instead of the full. Finally, look at the bling and decide if you want a 20th anniversary medal for the full.
From a physical standpoint, either is doable at this point in time. Frankly unlike other sports, there is not really a pecking order of progressions that one must do in the running world.
As far as planning days at disney, we tend to not arrive until Thursday or Friday then stay later into the following week. For us it is all about having the parks to ourselves and avoiding the hanger-onners from New Years.