If you select “I copied it” the following setting in the target Virtual Machine .vmx configuration file are changed.
Which makes the copied Virtual Machine appear uniquely different the the original however it’s not totally unless you make the appropriate modifications at the Guest OS level as well however to Fusion it appears unique.
If your running copies of the same Virtual Machine at the same time on the same system there would be network connectivity issues if the “ethernet0.generatedAddress” was not changed. If you’re just moving or running a single copy of the target Virtual Machine at a time then you may want to choose “I moved it” to avoid possible activation issues with the OS and or Software that registers the Ethernet MAC as part of its checksum.
More info: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1541