For Europeans VDR is definitely better then MythTV. It was designed specially for that area. VDR has much more plug-ins. But MythTV has a strict client-server structure which allows to use portable devices as its clients (N800, for example).
You can choose VDR or MythTV on the Media director page (Software modules, I guess). But I'm not sure how it'll be after changing PVR system. Most probably you'll have both MythTV and VDR running on your core.
Michael - VDR is designed for DVB generally, and therefore is better suited for practically every country with digital transmissions in Europe, Africa, Asia, Australasia and most of South America. DVB is a worldwide standard, ATSC is almost exclusively North America. If you only have ATSC available then Myth is the only option with LMCE, anywhere else you can use either, but VDR is still clearly much more transparently integrated with LMCE...