We've eaten at both for dinner on our last two trips.
My kids much prefer CM. Though I'm personally not crazy about the food (it's typical Disney buffet stuff), it's decent and very kid friendly. The atmosphere is silly and Mickey and friends are everywhere which is always fun. I'm not sure how much your kids like characters at their age, though.
WCC has a lot of fans, but we didn't like it last time. We found the antics had gone down a lot since our previous visits, and our food was not that great this time. My kids didn't really like it.
When we were planning which places to return to on our upcoming July trip, all three of my kids requested Chef Mickey's. No one, not the kids, DH, or I mentioned WCC.
But again, you'll hear lots of different opinions on these boards.