This is an endless debate, often discussed in the forums.
I use Canon, so the choice for me is 60 or 100. The difference is that, while both offer 1:1 maximum macro magnification, the 105 achieves this from a greater distance. This can be advantageous with skittish critters, but you'll have more water between the lens and subject. The 60 will provide the same magnification, but you need to get closer to the subject to achieve it. In clear water, the greater lens-to-subject focus distance of the 100/105 isn't usually an issue. As visibility declines, though, you'll want to get closer to the subject and this is where the 60 is advantageous. The 60 also can be used more easily for medium-range fish pictures, as the 100/105 will often give you too much magnification. or require too much distance from the subject.
The 100/105 is likely preferable in situations where it's difficult to approach the subject. The 60 is usually preferable where viz is more limited, you can get closer to the subject and you might want to get some more medium-range shots on the same dive. With the 100/105 and longer lenses, you are pretty much committed to maximum macro.
From what I have read in these forums, the common recommendation is to start with the 60 and consider adding the 100/105 later if you are bumping the limitations of the 60.