The deal types in these seasons seem to vary so it can be hard to compare. Free Dining in Fall is definitely better than the Stay Play Dine deal they like to do in winter/early spring.
In theory, the deals for fall typically are better because of the crowd patterns:
Fall has the following things working against it: It's still warm through much of the east coast, Kids just went back to school or are preparing to go back and people don't want to disrupt them getting back in this routine, there are no major holiday weekends. There are a few draws in fall though with Epcot F&W,
MNSSHP (Sept/Oct) &
MVMCP (Nov/Early Dec). Also I feel the weather in fall at Disney is better and more predictable (it may be hot, but it's unlikely to be cold)!
Now Spring has MLK, Presidents Day, Easter and Spring Break season which are major travel dates with large portions of the country out of school. The Northeast is cold and snowy and people just want to get OUT. People are more willing to pull the kids out of school mid-year. Weather in Jan/Feb can be unpredictable including lows in the 30's (brrr!) but once you hit March it seems to be a little more predictable.
This doesn't necessarily mean fall deals will be better, but to me the facts suggest they should be. Disney is slowly lessening the promotional amounts though each year as attendance continues to be strong. Also the room types and availability has gotten more limited, it seems now Disney is using these promotions to steer people towards certain dates and room types that they know they have a little more trouble selling.
Good Luck with your trip planning and I'm sure whenever you go it will be an awesome time! Don't dismiss free dining either if you haven't spent some time looking at it. For our family of 4 staying at VALUE or MODERATE resorts we can beat paying for the dining plan BIG TIME on our own but no other discount has ever saved us as much money as free dining did. Your makeup of adults and kids in your travel party and their eating patterns may vary though (and if you're planning to stay DELUXE then you probably can ignore this statement). For us as an example - a 25% discount on the room may save me $500 on a 7 day trip but I know paying NOTHING for dining will save me at least $700 on that same trip - assuming $100 for food for our family of 4. For us I would find it hard for Free Dining to not win out.