There really is no universal answer to the FD vs Room Discount.
Although generally, FD will be better.
IF you are planning to get
Disney Dining plan, then FD is pretty much always better.
The only way it wouldn't be, is if your savings on your room per day are great then the 50/55$ per day per person that Dining costs.
So if its a 30% discount, if you room is 300/day, and you have 2 people, you are still better off with the free dining. Increase beyond 2 people, and FD will be better.
Now, that all depends on how well the Dining plan works for you, even though we have the DP booked for our next trip, I
know it doesn't work well for us, but my wife likes to know its all taken care of ahead of time.
For us, we are better off getting APs, and a TIW card. We often split a QS meal for lunch, and often don't get the most expensive thing on the menu for dinner, which you basically have to do in order to get your monies worth.
So, there are combinations of Room discount, food discount or OOP food, that actually can save you more money then Free Dining, BUT, as a general rule, FD seems to work out best for most people.