Hi,
vSOM is packaged with vCenter Operations Manager Standard - regardless of which edition of vSOM you purchase. The vSOM editions (Ent, Ent Plus) a different only in the version of vSphere, not vC Ops.
So, if you already own vSphere Enterprise Plus and want vC Ops Manager Standard, then you're probably better off going the upgrade route. This is because another difference is that vSOM is sold per CPU, which covers the vC Ops Manager entitlement as well.
If you just bought vC Ops Standard, you would be buying per VM - and if you have a lot of VMs it would likely be more costly to do it that way.
Hope that helps.
