I would not finance the purchase. If you want to pay with the Marriott Visa, pay the amount in full on the card (you'll get lots of points that way!) and then pay off the amount completely when you get the bill.
I bought directly from Marriott even though I knew about the resale market. There are many very good reasons to buy directly from Marriott that you should research on this board and there is not a one-size-fits-all answer as the above posters might tell you. A poster did mention that if you get a lot of incentive points to buy, which i'm sure they did offer to you, that would be a prime reason to purchase from Marriott. The cost of that vacation alone is worth the price difference. If you purchase on the resale market, you obviously won't get those points and you'll lose out on an amazing vacation. Also, you'll be able to exchange your unit for points if you decide to take a trip to Paris or Italy where there are no timeshares. We loved that flexibility. Then again, that may not matter to you and if so, you should buy resale. Good luck in whatever you chose!