Hello, Sylvia! :-)
In order to determine if you'll get the most out of your trip by adding a
Dining Plan, take a look at the PDFs which are posted on Walt Disney World's website. There, you'll find an explanation of the benefits of the various plans and can compare what each plan offers to how you expect you'll eat once in Walt Disney World.
For example, do you foresee yourselves grabbing Quick-Service meals, except for the Spirit of Aloha Dinner Show? If you choose the regular Dining Plan (as opposed to the Quick-Service Dining Plan or the Deluxe Dining Plan) each of you will be entitled to one Quick-Service Meal (entree, sides, dessert and non-alcoholic beverage), one Table-Service Meal (one entree, sides, dessert and non-alcoholic beverage) and one snack per night of your stay. You will also each receive a resort-refillable mug, good for the duration of your stay. (I just purchased one during my stay in March, and the cost was about $15 including tax.) For a rough estimate of the cost of food at the parks, check out the unofficial site, allears.net, which is really great at updating posts with menus as well as pricing of all of the Walt Disney World Resort dining locations. Add up what you think you might spend on meals as well as the cost of your Dinner Show, then compare that total to the cost of adding the Dining Plan to your reservation. Keep in mind that your dinner show will "cost" 2 Table-Service entitlements, so you'll still have a third entitlement available to you, should you choose to purchase the Dining Plan.
Let me know if you have questions about any of the restaurants or dining options available to you, whether you choose to add a Dining Plan or not; there are so many choices, it's half the fun of planning a trip to Walt Disney World!