Not sure when you last went, but for the most part, to eat at a TS restaurant, especially at a "normal" meal time, you will need an ADR. Which will tie you to a reservation if you are doing the DP or paying out of pocket.
Look at the places you have an interest in eating at. Check out the menus, and see how much a meal you would eat would cost you. Add up the totals, and see if if the DP or the room discount comes out in your favor. If you wouldn't eat all the food the DP would get you, then you most likely would do better off to pay out of pocket. Especially if you look at the meals that take 2 credits, they don't usually justify two meals. I.E. a 1 credit dinner might cost you 25 out of pocket, but a 2 credit might only cost you 40. So you lose 10 dollars worth of food (or whatever the real world numbers work out to) by dining at the 2 credit restaurant.
I personally see no benefit to the dining plan unless your menu choices will break even. Most dining plan numbers I have run come out to no real discount. It sounds like you have already done the math to see which works better for you, but that is really the deciding factor in my opinion. Some families do much better to take free dining than a room discount due to family size.