For corals the only gain you'll get going from the 55 to th 105 is that you'll be able to get 1:1 reproduction a little further back in your tank. For fish, it might help to have the VR and a fast focusing lens (haven't compared the two so I don't know if the 105 is a faster/slower focuser than the 55).
You'd probably have more fun adding an extension tube or a teleconverter to your kit.
Here's a shot taken with an extension tube. The subject is right up against my front glass so I can get physically really close to it.
