If you are as big a penny pincher as me, you will definitely want to eat off site! It all depends what your budget is and how frugal you are. I'm a cheapskate most of the time, I guess!
I couldn't bring myself to pay the prices at Epcot for dinner -- any I didn't care if I sat down to eat or ate fast food -- both were way too much for me! We did get a small popcorn for a snack, but brought our own snacks otherwise. The only restaurant that was fairly reasonable was the Mexican one at Epcot. The others were $18 and up per person! If it's just 2 adults, that's not THAT bad, but if you have 4 kids, you'd spend $100 a meal compared to $30-40 at a buffet!
Even the "fast food" at Epcot was high. It was $5-6 dollars, I think, for a bratwurst. We brought snacks, and left at 7:30pm for a late dinner off site.
(I loved Sweet Tomatoes, too. There are coupons, so make sure you use them when you go there.)
When we did eat lunch at Universal Studios, it cost $20 for my DH and I to each have a burger and fries and SPLIT a shake and a cookie. That was at a counter where you carry your own food, etc. The prices were comparable at Epcot. (We didn't go to the Magic Kingdom this time.)
Hope this helps!