While I will admit that SAM is a particularly awesome product, it's primarily geared towards monitoring physical and virtual Windows and Linux/Unix hosts, and the applications that run on them. SAM 5.5 included network interface monitoring via WMI, but does not include SNMP interface monitoring required for routers/switches/wireless access points/etc.
Speaking of Wireless, SAM does not include NPM's wireless monitoring capabilities, F5 load balancer support, Cisco UCS support, or include the Universal Device Poller. SAM does include an SNMP monitor, but it's not nearly as powerful as the Universal Device Poller. NPM also includes support for monitoring vSAN/fibre channel switches, Cisco EnergyWise, as well as routing tables and multicast introduced in NPM 10.5. Those features are exclusive to NPM.
SAM 6.0 introduces a whole heap of awesome, so I tend to agree with simonplexxi, that you should look at both. Depending if your needs are more server or network based, you may find one product better suited to your specific needs, but if you have responsibilities that span both disciplines I would definitely recommend both.
