Hi,
it depends on what do you mean by saying "NMS" - some people understand this as "Network Management System", while others might think "Network Monitoring System".
If you need management, then I would suggest you to use the system of the vendor you use. So, if you have Cisco equipment, look for Cisco management system.
If you need monitoring, it depends on your needs and money available. For example - you want to get experience and full control on monitoring tools, protocols, practices and have anough time and patience to study, then take any open-source system and give it a try - Nagios, Zabbix, Friendly Pinger, Nedi ;) etc (https://en.wikipedia.org/wiki/Comparison_of_network_monitoring_systems). As from my experience, I tried many of them and stopped with Zabbix. It is hard to start, and I spent about a week to make it ping one router and send simple reports to e-mail. But after you will get the basics of Zabbix, you will feel the power of this and make configurations by intuition. More advantages - this is open-source, runs on Linux (which is free and open-source too), it has good community, support many data storages, does not require to dig into internal scripting, many people share ready-to-use templates, so most of the cases are already solved for you. As for Nagios and Nedi - they are easy to install and make some basic things, but if you want to go deeper and make sophisticated monitoring dependecies, IMHO these systems wil become real headache to manage.
Again, if you have money and do not have time/passion to study, buy SolarWinds, pay for support and ask them to configure everything. If you want to get exeprience and knowledge - take Linux + open-source NMS and keep reading RTFMs.
Good luck!
