We've done both and out of my group of 11, most 'prefer' breakfast, but everyone (except my teenage niece) liked lunch as well. Most everyone in our group really likes the stuffed french toast for breakfast~ so that puts it over the top for them! Either way the food was good.
Now, if you can possibly do a breakfast before the park opens, I say do that for sure!!! Getting in the park before it opens makes for great picture opportunities~ very few people in the park at all. We've done it several times and wish we could this year (going Christmas week when the parks open at 7 am anyway, so...). We always get one of the the first time slots available~ which always worked out that we were finished with the breakfast 5 to 10 minutes before park opening~ putting us at the Fantasyland rope drop~ one of the very first in Fantasyland~ that is wonderful! IF that is possible, I say do that way over lunch! You don't lose park time eating, get in the park when it is almost empty for a very neat experience and picture opprotunities, and the first group in Fantasyland. Can't beat that! But if not, either breakfast or lunch is good!