Wendy, this is a showdown for the ages. Brace yourself.
'Ohana is a little more laid back thanks to the ambiance of the restaurant. The
Supercalifragilistic Breakfast at 1900 Park Fair at the Grand Floridian is more high energy due to its table spacing, size of room and foot traffic.
Character-wise at 'Ohana, you'll be seeing Lilo and Stitch for sure. Pluto and Mickey are usually there, and Goofy and Minnie sometimes show up too. At the 1900 Park Fare breakfast, you'll get Mary Poppins, Alice in Wonderland and the Mad Hatter. Tigger and Pooh have also been part of the entourage every time we've been there.
Breakfast is family-style at 'Ohana and buffet-style at 1900 Park Fare. As such, you'll get a bigger selection and variety at 1900 Park Fare. You'll definitely be full when exiting from either one.
So which one should you go with? If you'll be seeing Mickey and Pluto elsewhere on your trip, then I'd think that would decrease some of the appeal of going to 'Ohana (unless you're super fond of Lilo and Stitch). On the other hand, Mary Poppins, Alice and the Mad Hatter appear less often throughout the parks. This gives you the opportunity to get some photo ops with them. And, if you have little kids who are apprehensive about characters in full body costume, kids usually show no aversion to these face characters.
Which would I choose? 1900 Park Fare.