The two products are for different purposes. In my organization, NPM monitors available and uptime and errors and status of devices and ports. But NPM doesn't monitor the status of applications or internal processes inside servers, or the web pages that users access. That's where SAM comes in.
I started with NPM to get an initial idea of the status of my switches and routers. Then I expanded it to look into port errors and bandwidth utilization. Later I began monitoring servers with NPM's Quality Of Experience solution, by adding servers into NPM using WMI.
NPM doesn't provide the in depth detail for servers and applications that SAM provides. I'd recommend getting your feet wet with NPM, then advance into SAM to get the best performance for your clients from your servers and applications.
But neither is "best" when compared to each other; each one does different tasks, and has many strengths that the other does not have. In fact, they're so different you might say comparing them is like comparing apples and orangutans.
