From start to finish it will take longer before you have the points at your disposal to use, but other than that I don't see that there are any negatives to going resale. We bought both our contracts resale and have been extremely happy and there is no difference between a membership bought thru Disney and one bought resale.
The nice things about going through Disney is that if you call tomorrow and sign the paperwork next week, you can pretty much start making reservations right away. If you need financing it seems they make it easy.
However, if you have other ways to pay for it and don't mind taking a couple of months to get everything processed and having your points show up in the system, resale is a great way to go to get something specific that you want (size contract, sold out location).