I can't be too much help, I'm afraid. Whenever I've dealt with similar things and had a choice, it's always been the case that one method worked (out of things like wdm and vfw) and others didn't. I did not really have a choice - I just used the one that didn't crash.
Technically speaking the question itself is not well formed. WDM and DirectX are not competing technologies as I understand it. WDM is a driver model, which allows a/v device vendors to create a low level driver that can be used easily underneath a number of media Windows APIs without having to support each of them explicitly. I.e. they allow generic high-level Windows MME, DirectX, etc... code to provide the high level interfaces and work in terms of the underlying WDM driver, as apposed to the driver itself providing those APIs directly.
That's my understanding of it anyway. So it would seem that if you had a WDM driver that that would be fine and that it would also allow DirectX control of the device as well since WDM drivers allow for DirectX access as well.