Sonotype Nexus or Asset Manager

Hi,
I wondered what are the benefits of using RAM over Nexus. A quick peek at the RAM manual says that it can be used to import an existing maven repository into RAM which can then be used by maven clients.
But with a nexus, the maven client would point to it and then the nexus would point to a public maven repository and get loaded as and when a request was made. Which seems cleaner.
So interested in opinions and experience of the community.
Regards,
John