The two different memory collection rules should be (automatically) used depending on whether 'VM Child Objects' is enabled - this is our topology option, of having a simpler more scalable topology with less objects, or the full set of child objects including a specific vRAM object for memory counters, vCPU object for CPU counters etc.
These rules should not change depending on whether there is an Ops Mgr agent installed....However, the rules change depending on group membership of VMs, the group is controlled by the VM property 'Child Objects Enabled'. So perhaps there is some problem with the group membership. Maybe the 'VMGuest Contains Ops Mgr Agent' discovery is a factor, because it does update some properties of the associated VMGUEST object....
We will investigate here in R&D and update you ASAP. Thanks!