They are pretty complimentary...
This might help you with NBAR
http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6558/ps6612/ps6653/prod_qas09186a00800a3ded_ps6616_Products_Q_and_A_Item.html
This is helpful for Netflow
http://www.cisco.com/en/US/products/ps6601/prod_presentation_list.html
Netflow has very specific applications that it can classify. Sometimes the 'true' determination is left for the netflow analyzer (NMS application) to handle.
Netflow provides details about network traffic, where you can filter the search by the IP
address, source/destination, port, AS... so you can isolate which user starts some connections.
NBAR is a classification tool for applications, more for upper layers (4-7), especially a few stateful protocols.