Purchasing the regular dining plan may be the best deal for your group if:
1) You plan to eat a table service dinner (not breakfast or lunch) every night of your stay.
2) You would like many of those to be character meals or fixed price family style options.
3) If you want to go to a la carte restaurants, a number of your party are likely to order the more expensive items on the menu.
4) You prefer desserts to appetizers (the desserts are included; the appetizers aren't).
If you don't fit these, the
DDP may possibly be a deal for you if you have several children under 10 in your party and they are OK with the kids menu or you are going to do mostly fixed price meals.
If you don't fit any of the above, and your main interest in the dining plan is the possibililty of saving money, you need to come up with an idea of where your party will eat on their stay, what they might order, and how much that would cost in order to compare it to the dining plan cost per night for your party.
If you also have a "convenience" interest in the dining plan because it's prepaid, you'll have to take that into consideration and balance it against whatever savings you might be able to realize, or not realize. (for example, if you do the math and you're going to just about break even DDP or no DDP, you can then decide if you want to prepay and order the courses the DDP requires you to order, or go OOP and order what you want - giving yourself the option to get appetizers without it being an extra purchase, or for children under 10 to order from the adult menu, if your party is so inclined.)