Only a few reasons to get the 100 over the 60, IMO....
1. You need the working distance (you shoot bugs and other things that shy away from cameras).
2. You want to have a longer prime lens.
3. You plan on a FF upgrade in the near future.
My 60mm macro is small, lightweight, very well built (makes my 17-55 seem like a tinker-toy), easy to hand hold at lower SS's, extremely sharp, very fast focusing and doubles as a great portrait lens. Its also seems sharper wide open than my 50mm f/1.4 is at f/2.8!
If you don't plan on getting a longer zoom in the near future, then the 100 mm macro will come in handy, but if you plan on getting a 70-200 or 70-300 then I would consider the 60mm macro.
Just my 2 cents.