One really cannot compare apples with oranges. There are actually two types of IPS systems:
Signature Based
Anomaly/Behavior Based
Most modern IPS systems are a combination of the above two, but most of their core detection functionality is based on pre-defined signatures. Netflow is a tool that can help achive the second of the above, as in baseline the 'normal' network traffic and then use that to do further analysis. It does not help much with protocol anomalies/exploitations like DNS/DHCP attacks, VOIP attacks, FTP attacks etc.
In the Cisco solution, you can send NetFlow events from IOS Devices to the Cisco MARS, as well as events from Cisco IPS Sensors,IOS IPS, CSA logs, firewall logs and it can correlate both to achieve the above two goals in an integrated fashion.
There are other products that focus on Network-Based-Anomaly Detection (NBAD) using Netflow/Sfow/Cflow, have a look at:
http://www.networkcomputing.com/showArticle.jhtml?articleID=163700677
However please note that they NBAD devices don't come that cheap :)
Regards
Farrukh