Is "interline" discount mean you get 40% off on your room?
There are a lot of factors - what level hotel are you staying in? If you are staying in a deluxe then I would think that that the 40% off on the room would be the better discount. However, that might be offset by the time of season. But I think even in the value season that 40% off a deluxe hotel room would be more of a value than the
free dining. Let's assume your hotel room in a deluxe is about $400. This is just a random number because during value seasons the deluxe hotels, depening on the room, can be as low as say $270, but during peak season and room category it could be as high as $500+. I'm using $400 as an example but you'll have to do the math to see what hotel and room category works for you. You can go to the Disney site and price out hotels to figure out your price. But...40% off of a $400 room is $160 in savings a night. That's more of a discount than FD. If you stay at a value though and you're paying $200 a night or less then 40% would only save you $80 a night and then FD would be more of a savings.
What level dining do you want? The basic
ddp is $111.18 a night for two adults. It's a little more during peak season but by just a few dollars a night. The deluxe ddp is $199.94 a night for two adults. If you do free dining and want the regular dining plan then you'd save $111.18 a night x's the number of nights you are staying. If you want the deluxe ddp then you'd pay the difference between the two plans and still basically save the $111.18 a night but pay the difference for the deluxe option.
Also, what/how do you like to eat? The FD can be worth it if you eat at character meals and table service for many of the meals and take advantage of the snacks. Otherwise it can be cheaper to pay for meals out of pocket. We are doing the deluxe ddp in January and have a signature restaurant every night and a TS for breakfast or lunch everyday. When I did the math the cost of the dxddp doesn't really save us a ton but it saves enough that it makes it worth it for the extra snacks and the 2% back on our Disney Visa Card in rewards.