OnClick vs OnClientClick

I've read that OnClick is executed on the server and onClientClick is executed on the client.  Is it better to use OnClientClick to do something like close a modal popup?
 If my understanding is correct then this would avoid a trip to the server to accomplish the same task.   This would be great for something that doesn't need to make a round trip to the server and back.  
Is this understanding correct?
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