It just really depends. Which resort, which room category, which season, how many people in your party, what is the discount percentage.
For example, we are booked at WL in January 2013. We are a family of 2 adults, 2 kids. Because it's value season and rates are at their lowest-- and WL is less expensive than other deluxes like the Poly -- even if a 35% off room discount comes out, we would still save more money with a free dining offer. If the room discount was only 20% (as they have been sometimes lately), the difference is even more significant in favor of free dining.
Last year we stayed at the Poly, again in value season. A 35% room discount came out and saved us a little more than our initial free dining booking. But if the discount had been less, it probably would have tipped back to free dining being better.
Now, if we were only 2 people in our party, or if we were traveling during a more expensive season, then the advantage would probably tilt back in favor of a room discount. The math is different every time depending in the details of our trip.