I'd say it matters more what you want to do with the points. If you think you're going to want to stay at the Poly all or the large majority of the time, you should wait and purchase there.
If you want to try different places throughout the system, and different locations beyond just Orlando, then buy an inexpensive resale option now- for less total points than you think you need. Then add-on a small contract at the Poly, to stay there with home resort priority every year for a couple days, or every third year for your full trip.
The latter is the option we went with, and we haven't regretted it. And if it turns out that the price and
point charts for Poly are ridiculous, then we will likely buy another inexpensive resale and be content to get the Poly at seven months on rare off-season trips.
The Poly was our first Disney deluxe stay, and it's our favorite resort in WDW. But if
DVC makes the math too terrible, then we will be content to stay elsewhere. Buying direct at recent prices is a whole different ball game than it was in years past, due to the very wide spread between the respective markets for most properties.