It was hard for me to decide the same question. Since the answer is different for every family, you have to sit and crunch some numbers.
What I did was go through and price everything separately. And I do mean EVERYTHING. I went to touringplans.com and figured out which park we were doing each day using their advice on which parks were more crowded. After I got my park days, I figured out which restaurants we wanted to go to using my PassPorter. Then I went to allearsnet.com and looked at menus. I tried to figure out roughly what everyone in the family would eat and wrote down prices on the meals. If you do this, try to be realistic. Don't add appetizers if you don't normally order them just to compare with dining plan. I then added tax and gratuity. I also added in snacks if I thought we might want one that day. All that and I had the price of food for us.
Next, I figured out what kind of tickets we wanted. For us it ended up being Park Hoppers. I used my mousesavers.com link to price the tickets through undercovertourist.com so I could get the best price.
Of course, I got the price of the room including tax. (I also checked to see if it was worth it to get AAA for the discount on the rooms or if there was a room only code that I could use. Since there wasn't yet, I figured it without)
I added all that together and got one price. I know that some of this may sound like an advertisment, but I wanted you to know where I was getting what I considered the most accurate and cheapest information. Someone who is kind of new at this might be able to utilize these resources, too.
Then for comparison, I used the package price including the dining plan, plus the cost of breakfasts (which for us is not much) and got another price.
I was surprised to find that for us the package price saved us almost $500 over 9 days just because of the dining plan.
I know that other families have found just the opposite. It takes awhile to do it, but in the end doing this really saved us some money. I wasn't planning on doing a package since historically, it has been cheaper to book things individually. This way convinced me.
I hope this helps.
A