The difference is $38.99 per adult and $10.99 per child...per day. There is no difference in the benefits of DDP based on whether you pay or don't.
You may find the restaurants more crowded than you'd like, and ADRs are a must. Some have reported less "magic" from the CMs during free dining, but I don't know if that's true.
Went there last year and had the free dining in our package. You simply show them your card tell them you are on the dining play. They have no idea if you got the free offer or bought the dining plan. CM takes the card at the end of the meal to deduct 1 TS credit per person. The recipt shows how may TS, CS and Snack credits you have left for your vacation. Real simple and loved it.
Dad Mom Bobby Kim
1995 Port Orleans
1996 Dixie Landings
1997 Caribbean Beach
1998 Dixie Landings
1999 Dixie Landings
2000 Dixie Landings
2001 Dixie Landings
2002 Animal Kingdom Lodge
2003 Dixie Landings
2006 All Stars Music
2007 Maybe Pop
The only other difference I can think of was last year a few restaurants changed their menus for the free dining period. Le Cellier removed the cheese plate and dessert sampler, and Spoodles eliminated the shrimp add-on. Spoodles also didn't allow the dessert sampler, even if two people both used their credits for it, though others have reported being able to get it outside of the free dining period.
Of course, these changes apply to everyone dining during the free dining period, regardless of whether you got it for free or not.
No difference as far as making reservations--as others have said, CM's on the phone and at the restaurants have no way of knowing whether you paid for the DDP or got it free through a promotion.