If on a budget, get a manual focus macro lens off eBay. I have purchased them from $75 to $200. You will lose TTL metering and of course auto focus. 95% of my macro shots, including fish, are taken with manual focus (set my new lenses to manual). All the different focal lengths have there place. I have Nikon Micros in 55, 60, 105 and 200mm lengths and use them all.
Like I mentioned in my first post, my fish room is tight. I could not use the 105mm and get an entire 6" fish in the frame, just can not back away from the tank enough without hitting the row behind me. That is where the 60mm comes in handy. Sometimes you are shooting a skittish fish and it is better to be back away from the tank a bit so it calms down, in this case the 105mm would be a good choice. If all you want are close up details of part of a fish or a coral then either one will work, with the longer lens bringing the subject closer. I still say the 60mm is more versatile, but that is just me.
For insects I always use a longer lens, the 105 or 200mm.
In my fish room, I could not take this shot with a 105mm lens. (Sorry, not marine fish - just getting back into the hobby)
Old photo of my fish room.
This was with my 105mm, get too close he flys away. This was also handheld. This one used AF which is why the focus isn't as sharp as it should be.