I have been told that there will not be a Vista Embedded but there will be feature pack add-ons to XP embedded as it stands currently to provide support for the new features. This makes a lot more sense to me than completely re-writing/ issuing the embedded system as it means that backward compatibility and remote updates of such enhancements can be pushed out to all devices, which could be very useful under certain circumstances.
Some things to consider when deciding between the systems is whether you need real-time systems, and how big a footprint you can support with your hardware. Also consider what sort of future applications you will be looking to support... it will become obvious what operating system type you should be going for in the end.