I want to monitor Cisco's newflows.
Which one of these tools do you recommend?
Both will do the job. NFSEN collects NetFlow flows using the nfdump tools, NTOP collects NetFlow and sFlow flows using nProbe, which means you can collect and process flows from Cisco, Juniper, Procurve, Extreme and a number of other devices. NFSENS is pretty much a dedicated NetFlow analyses tool and if all you're looking to do is look over flows then give it a go. NTOP is much more of a general network usage analyser with a NetFlow plugin. It'll give you all sort of network statistics and enable you to look at your flows, all from a single console. I'd recommend NTOP.