I have almost all of my servers set on snmp as the datasource and SNMP traps configured. I've done this because of firewall issues with wbem trapping. The SNMP setting works as good as WBEM. HP on the other hand tends to abandon the SNMP prototol on Windows systems. Unix only work with SNMP, devices only support SNMP and ESX later than 4.1 supports WBEM. I've only seen that system details are more complete on windows and ESX servers if set on WBEM. If you set your servers to WBEM, don't forget to set subscriptions from SIM. If you use SNMP as a datasource don't forget to set your SNMP trap in your SNMP configs on the servers that you want to receive traps from.
I see no real advantage in either options, just go with works for you. Regarding the WBEM discovery, check if the wbem port is open, or just for test sake disable the firewall temporarly.
