South Beach isn't to everybody's liking, but I think that you already suspect that a hotel with wide-open Points + Cash availability over the holidays isn't exactly desired by tourists. The HR is definitely not Miami Beach!

But it all comes down to convenience (for what you plan on doing) versus money. Christmas is a long time away. I would book something refundable, then check for award availability every once in awhile. Also check whether some of your dates are available on points, book those now, then keep looking for award nights and/or fill in gaps with paid nights.