Well, sounds like you are using your other timeshare privilege + $129 to get a week at Marriott. So you should factor in the cost of that privilege when you calculate the total cost. Say, if you spend several hundred dollars more to rent from DVC, then you (or your husband's dad) can have a separate vacation at sunterra (or wherever). But if you don't expect to be able to use the sunterra thing, then personally I would take the $129 deal.
But there are always people who insists to stay on site for whatever cost, for the convenience. This is especially useful with little kids. Oh, staying on site also maximizes the use of Disney parks (sometimes they open an hour early for on site guests, free parking, free transportation through the resort, etc.) If you have only, say, 4 days, and you want to see all the parks, you'll see more of Disney by staying on site.
So your decision will basically come down to your situation. What you want to do and see (Disney only or outside of Disney), how important is convenience, whether you plan to have another vacation to use the sunterra club privilege, etc.
