One specific thing to consider - at Whispering Canyon, most of the meat is smoked. Since my wife doesn't like smoked meats all that much, she didn't enjoy it especially. At 'Ohana, I don't recall any of the meats being smoked.
On the other hand - Whispering Canyon has the only milkshake with free refills I've ever seen.
Personally, I really like both, but I've been to WC about five times, so I probably won't go back on my next couple visits. At the moment, 'Ohana is up very near the top of my favorites list.
Side point - it's a good thing the "Fabulous Disney Babe" hasn't read this thread, or some of our posters would be sent to the corner for a time out - any time she sees this restaurant called "O'Hana's", we immediately hear "I didn't know the Polynesian had opened an Irish pub." (Or sometimes, she's just a bit less polite about it.) So - you've all been warned!