The best choice depends on several things:
- your network size and complexity, esp multi-vendor management needs,
- whether or not you want servers and applications as well to be be monitored,
- your ability to customize and configure free (open source) vs. wanting a vendor-supported product with easy out-of-the-box setup,
- etc.
SolarWinds certainly offers a strong product suite (NPM plus NTA for the functions you mentioned) although large setups can get pricey (I have seen some US$ 6 figures). For many small to medium enterprises it is a good solution and it offers nice integration pay as you grow.
On the open source side you can put together a solution based on several tools. For instance, Nagios + Cacti + ntop + RANCID all combined give you a lot of useful information. Cost of ownership may or may not be higher as you do all the integration work yourself.
Some people speak highly of What's Up Gold although it's a more basic product. Also, ManageEngine's tools are well-regarded.
Of course an all-Cisco shop is best served by Cisco's offerings.
