There is no specific document comparing the products. It probably wouldn't be worthwhile at this point since the currently available version isn't an accurate depiction of what they will be releasing. 

With regards to Blend, the control can be used/designed within Blend. I'm not sure if there is a specific concern you had. It also doesn't have the same issue with commands that the original MS Ribbon had whether you were required to use their base command class to provide images, etc. With regards to M-V-VM, it would probably be best to have a layer in between the control and the vm. For example the tabs collection is not currently bindable. It may be possible to add this type of support in the future but it may be better suited to be managed in a separate layer (e.g. a prism region adapter). The xamRibbon supports customizations (e.g. adding ribbon groups and tools to the qat) that make it difficult to allow creating via items control type container generation around a data item.