I've used both Nagios and SolarWinds, although I haven't messed with nagios as much as
solarwinds. Solarwinds had some nice features for monitoring and I
learned a lot about it in the short time that I actually worked with it.
Nagios just seems to work and when a server is down, I investigate.
Solarwinds seemed to have more issues but that could have been because
it was running on a server 2003 box and possibly old hardware where as
the company I'm working at now runs nagios on a linux box with some
decent hardware. Again, I haven't delved deep into Nagios and it's
possible that what I'm looking at is just a webpage front end to nagios
that not everybody uses but it's still pretty nice regardless.
Not sure if different environments have different prefferences but the company I was working at that used solarwinds was an ISP. The company that uses nagios is a web hosting company. I've also seen a linux admin at a previous job use nagios so it may be that nagios is more popular among linux if not servers altogether.