FWIW, back in the dark ages, I always found myself reaching for the 100 mm macro over the 55 mm macro simply because the extra distance between the lens and subject made life a tad easier, IMO. Stopped down to f/22, the DOF difference of the longer lens never seemed noticeable. Might be a factor in your decision, particularly if you're planning to shoot wee beasties.
"Keeping shooting will do more for the excellence of your photography than most any camera will!" JimmyD, PGS Extraordinaire.