Wich macro lens ?

[dc3]

I just got an OK from my wife to buy the Canon xTi 400, probably because she doesn´t know that I'll need lenses that are more expensive than the camera itself.

Selecting the right lens for everyday shooting is difficult, but my main interest is in macro photography. So I probably will get the 100 mm Canon lens, but I wonder why everybody recommends 100 mm. I can understand that 60 mm Macros are to small but there are 150 and 180 mm Macros available, but for some reason no one seems to use them for Reef photography. Is it because they are only suitable for pods and smaller things or what ?

Gisli