If it's a one-day ticket, I'd buy now so that I could avoid the price increase AND make sure I could book my park reservation for Epcot. I know that the one-day tickets will include the park reservation, but what if Epcot books up for Dec. 17 before you buy that ticket?
NB: I have not taken the time to look up the park availability calendar for Dec. 17, and I will admit that Epcot doesn't tend to book up, but why take a chance, especially when the ticket will be more expensive on Dec. 8 than it is today?