We've done both twice!!!! I prefer the breakfast. Here's why: Even if you have the earliest lunch, it is still very loud and chaotic.
When we have done breakfast, we always get a PS for 8am (on non-ee day). Imagine this: You walk up an EMPTY Main Street, first thing in the morning. You arrive at CP-it's quiet and the morning light is just streaming through the windows. The buffet is scrumptious (and cheaper than lunch), every one is mellow (since it's still early) and the waiters and characters are "fresh". You are done with breakfast well before the park opens and you saunter over to rope drop. The first time we were on the first Dumbo ride and the second time my niece and I were the "first riders" on Space Mountain-just like I promised. She even got a certificate. After this, you won't want lunch until around 2pm!