The two serve different purposes. In fact, then NM-HDV-1E1-30 is just a bundling including a NM-HDV module and a VWIC-1MFT-E1.
The VWIC-1MFT-E1 is the access mechanism. It allows the DSP resources to access the channels on the media. The VWIC is just about the only device you can put in a router that will allow that, so either way, you'll need that.
The NM-HDV is what houses the DSP's. The DSP actually packetize and de-packetize the channels and allow the whole VoIP thing to work. However, there is another option here. There is the AIM-Voice-30 module on 2600, 3600, and 3700 series routers. Also, 1751 and 1760 routers have slots for DSP modules (called PVDM, the same modules on the NM-HDV) directly on the mainboard of the router. Both of these alternate solutions allow a VWIC to terminate voice channels in a WIC slot.
Now, as to which to use? That depends. If you need the NM slot on a 2600 series, then you would need the AIM card. If not, *I* would stick with the NM-HDV, simply because I'm more familiar with it and it's been around longer. The AIM module is still fairly new. Another thing to consider is that the NM-HDV can be installed with only enough PVDM's to support the channels you need, but can still be upgraded to support up to 2 E1's. The AIM-Voice-30 supports 30 channels out of the box and cannot be upgraded.
Hope that helps.
