Last year we ended up doing a split stay at ASMo and CBR. My children (then ages 6 and 8) liked both places, and other than the pool being better at CBR, we didn't notice any big difference. In fact, we liked the food court better at the values (AS or POP--felt like more variety) and found the walk to the bus stop (at the resort, not the parks) and wait/travel time to be comparable. For us, the additional $ was not worth it for a mod. This year I am booking our entire trip at a value so that I can say yes to more extras which for us are more important than a larger room or resort amenities that we don't end up using. So if my children want popsicles twice a day every day I can say yes
If there is a balloon or toy they really really want, then I can say yes
And if I want to have an adult beverage lounging by the pool or send them on a pirate cruise then I can say yes
For us the $400 would go a lot further in making a more relaxed and enjoyable vacation.