They are not really different controls, but one control with set different ContainerLayout property. You can read more about the Container here.
But you're also correct that runtime there is no difference between the two containers. They only differ when initialized - the one will be sized according to its children, while the other (Fixed) won't.
Please also note that this Containers logic is subject to change and the Containers in the upcoming new version will behave much better.
Please let us know if you have any further questions regarding the Containers.
the Telerik team