Comparing the forums, support, Nagios is very difficult to set-up. It has no good forum support, the official support channel is a mailing list with one thread. Zabbix is hard to install but you can learn it fast and you can end up with a more complete tool when is done. Zabbix combines several tools that are left out of Nagios. You can try and integrate Nagios with things like cacti, or rrd, but those integrations often may fall apart when you update Nagios or the other tools. One is faster and one does more. Also Zabbix is better for data collection and one centralized solution but on the other hand it misses soft/hard trigger states which Nagios has.