The WoC Picnic provides the same fastpass as the ones you get early in the morning. Whereas the WoC Dining Package provides the preferred viewing area.
In both cases, however, your viewing experience is also directly related to when you showup to the queuing area before the show. However, in our case, we've shown up at the last moment for the Dining Area and had great viewing.
The value is primarily in the meal, followed by not having to show up early to pickup tickets. Things which have diminished value (for Dinner only), and apparently YMMV, is things such as, having to line up early, as well as the viewing spot itself.