On the whole, I would tend to lean toward TiW (if you qualify) over DDP. With caveats.
With TiW, there's a lot more flexibility involved -- you don't have to use a set number of a certain type of meals, feel like you have to order top-end items and desserts every time just to get to the most value. And if you like an adult beverage with dinner, TiW covers that, too.
The drawbacks are that you would need to use the card enough over the period that it is valid to make it worth the up-front cost, and that some DDP locations don't take TiW (but then again, the reverse is also true). TDC Nala also mentioned the blackout periods, which can be an issue depending on when you're traveling.
All else being equal, the only reason to choose DDP or DxDP over TiW is if the dining plan exactly (or at least, really, really closely) matches the way you really want to be ordering.