With those ages, I'd skip Epcot. My kids like the rides, but WS is kinda blah for them.
Really - just assess your kid's interests and decide based on that. With a 12 and 9 yo - any of the parks could be great for them. And it also depends on how often you go. I know a kid in that range that goes often enough that she would go for one day at MK - just to hit more parks. She hates skipping a park. But she can skip "some attractions" during a day.