I always have a TiW card. But, still...every single trip I go on, I still sit down with pencil and paper and figure out my dining costs both ways.. I pretty much know where and what we'll be eating. Sometimes I go alone, sometimes with just my 17 y/o dd, sometimes dh goes with us..it varies.
Figure out where you'll want to eat (and what). Then, figure what the cost of that would be out of pocket, plus tips. Then, figure what your cost would be with the
DDP..don't forget to add in the tips for each sit down meal..many forget to do that.
When I figure what my dining costs will be using the TiW card, I just don't add in the tip....it's a wash for me. And usually, the way we eat, the TiW card comes out cheaper. I bought a new one for our family trip on July 31. It has paid for itself now.....and I still have 9 or so days to go at WDW before the card expires for the year.
If the DDP works for you, you can still use the TiW card for adult beverages, as well as anything not covered by the DDP...but it may not pay for itself nearly as quickly.