I have to say as someone that has used the DDP a lot over the last 8 years - it used to be a great value, then it was a good value...now it is of marginal value. If you eat at a ton of character buffets, it can work well for you, but overall it is NOT something I would recommend unless you are getting it at a discounted package rate or as part of "free dining". Neither of those apply to
DVC of course. Working the DDP well, and you can perhaps save some money. But it really depends on how you use it.
TiW is much easier to calculate. You save 20 % off certain restaurants. IT costs $100. If you are going to eat at restaurants that take it (And realize that much of the QS restaurants in the parks DON'T take it. ) and spend at least $500, then it will save you money.
What many people forget is that DVC members get a 10 - 20 % discount at many restaurants in Epcot (Chefs De France, Teppan Edo, Biergarten, Tuto Italia,etc), DTD (Raglan Road, T-Rex, etc) and at the DVC resorts (most of the Boardwalk restaurants, Sanaa, Citrico's, Artists Point,etc) . The full list is at the DVC website. This should also be factored into a TiW purchase. For instance, if you use your TiW card at Tutto Italia for a $100 meal, as a DVC member you get 15 % off anyways, so TiW only saves you an additional $5.
But overall, I would say the TiW is better in most cases.