I've done both - and liked both.
I prefer lunch. Lunch has always been served piping hot, and I can pick what I feel like at the time. The menu variety works for the folks I typically travel with and we feel like "it's fair pricing" for what you get in comparison to other Disney choices.
Dinner - I can't eat as much as they serve you at once, so by the time I am ready to try something that didn't make it on my plate the first time or I want seconds on, it's cold. (Hence one reason I don't love O'hana, either.) One person I travel with LOVES this meal - favorite foods, all-you-can-eat, no decisions about what to order. The food is fine - but it's prepped for the masses, seasoned for the masses, and just doesn't wow me. I feel like the lunch choices are more interesting, taste better (and prepped for slightly fewer people since there are more choices).
To summarize:
If you want your party to have choices - lunch.
If you want the all-you-can-eat and your party likes the menu - dinner.