Security Basics: Sniffer and Network Analyzer - which one?

["WALI" <hkhasgiwale () gmail com>]

Hi

Following on a related conversation I have been thinking to ask this query 
for
quite a long time but kept delaying it, so here I am:

Are the three Open Source products http://cacti.net/ , OpenNMS and NTOP
catering to different needs?

A white paper published here: http://www.techsupportalert.com/r1525.htm
doesn't even list any of these three on it's Top ten but they are most heard
in the technical forums.

Finally, within my LAN, that comprises of 10 subnet VLANs and spans across
10 floors, what would be the best way to incorporate any of these?

Pls advise!