We love both... but I'd say go with 1900 Park Fare. More variety in characters - Mary Poppins, Alice, Mad Hatter, Pooh and Tigger.... and definitely more variety as far as food.
Ohana we love also - but for us, mainly because we LOVE Stitch and Pluto there (Mickey & Lilo also there). We really enjoy the food, but just think Park Fare being a buffet you have many more options to pick from.
Ohana it always takes us a while to get seated, Park Fare, haven't had that issue. I don't think that should matter though, seeing you said you have the earliest reservation.