I don't think you could go wrong with either. I have been to 'Ohana multiple times and our experiences have been very good to great. I have never been been to WC, but close friends went last week and rave about it.
What type of food and experience interests you more: a playful waitstaff, western theme, and comfort foods...or entertainment during dinner and games for the kids, polynesian theme, grilled meats/wontons, and that incredible bread pudding?
Where would you rather spend your time beforehand/afterwards? Both resorts are beautiful and have lots to offer.
As someone mentioned before, keep in mind that 'Ohana can be kinda crazy during the dinner hours. The upstairs area at the Poly where people are waiting (and waiting) is crowded and frustrations can run high. Saw this myself last week and was so glad we were just passing through to get to the outdoor bar! For an 'Ohana meal, I'd schedule something earlier or later to avoid the mess.
I would also consider travel as a deciding factor. Is either one more convenient than the other?