As others have said, I don't know if the OP was referring to booking a breakfast or a dinner (or in the case of Tusker House a lunch and maybe she's not aware 'Ohanas doesn't serve lunch). The food for breakfast at both locations are incredibly similar, though there is more of a selection at Tusker House. I'd choose TH because I'd assume you're at least spending your early part of that day, if not the whole day, at Animal Kingdom, so why travel more than you have to?
If we're talking dinner, the one thing Tusker has over Ohanas as an advantage is selection, especially for kids. I'm sure if kids didn't like what Ohanas had to offer, they'd whip up something that'd appeal to them (in fact according to allears.net "ALTERNATIVE CHILDREN'S SELECTIONS AVAILABLE UPON REQUEST"), but with TH, you already have it. All the usual stuff kids would want as well as the opportunity to sample the adult offerings (which while not necessarily "authentically" African is more authentic than the breakfast offerings!). So if the kids in your party are picky eaters, TH might be a better way to go, but perhaps the best way to make the choice is the same as breakfast - what do you think you'll be doing that night? If you think you'll be having as late a dinner at TH as possible, essentially closing the park, then that makes sense. If you'd rather have an earlier dinner and then maybe go to MK for fireworks (or a late dinner where you can see them WHILE you eat) then go to the Poly.
Personally, I prefer Tusker House, but if my plans for dinner included going to MK afterwards, I'd choose to eat in a monorail resort, and Ohanas would probably be high on the choices.
And if they don't know how to spell the 'Graham' in 'Graham Cracker', then Lord, help them! And me, too! 
