I do not think that the 9th will sell out, that is a Tues. night and should be a light crowd. If a Fri. night was your only opportunity to attend, than I would buy in advance for a Fri. night party. Fri. nights tend to sell out.
I would wait until the 8th to see what the weather prediction was for the 9th. If buckets of rain are predicted there is a good chance that the parades and fireworks will be cancelled (if the rain actually happens) at your party. Buying from your hotel a day in advance still gets you the pre-sale price. Perhaps even the morning of would, but I'm not sure about that. You could go on Thurs. night instead (11th) if weather is looking bad for the 9th. Is the 7th (Sunday night) not an option for you? Sunday nights tend to be quite uncrowded. If Sunday was great weather I would go then, never know what may happen the rest of the week.
We went to WDW the first week of Dec. 2002. I had purchased our MVMCP tickets way in advance. We just got lucky that I had picked the "right night", Dec. 3rd (tues.). The weather predicted for the week picked Tues. night as a good night. Another night that week got washed out, no parades or fireworks. I will always wait until the last min. to buy my tickets now, unless I can only make it for a Fri. night (buy those in advance). Happy planning.