I'm not sure that they're specifically mutually exclusive, but the difference tends to be that WMI is going to be Microsoft only and SNMP is generally everything else. SNMP is used for Linux based operating systems as well as networking devices.
Windows can do SNMP, but that would require additional configuration vs what is baked in with WMI.
Here's some additional information you may find useful:
This is an excellent source for customers who are new to SolarWinds: https://support.solarwinds.com/Success_Center/Smart_Start_Onboarding_Program/New_to_SolarWinds It provides install instructions, best practices in the getting started guides and links to information on SolarWinds Academy product training (included when you are under active maintenance).