What is the difference between two configurations?

[MNenashev]

1. 2821 with VWIC2-1MFT-T1/E1 + PVDM2-32

2. 2821 with VWIC2-1MFT-T1/E1 + PVDM2-32 on NM-HDV2.

What is the difference?

When should I use NM?

[Hin Lee]

You would order the second one if you need more voice ports.

http://www.cisco.com/en/US/products/ps5854/products_data_sheet0900aecd8016fa68.html

[Chris Deren]

The difference is that the ISR router have PVDM slots on the motherboard. The 2821/51 have 3 slots, if you need more PVDMs then you can get NM-HDV2 card with PVDMs. On the old 2600 routers you had to use the NM cards in order to get PVDMs.

Chris