But that's a disadvantage if I'm trying to take pics from just a few feet, isn't it?
The 100 mm is better if you're working outside with wild animals, but the 60 mm might be a better choice for inside work with small pets (like geckos)?
Depends just how tame your geckos are.
Allow me to explain this in laymans terms....
With the 60mm Canon Macro, to get 1:1 Magnification (True macro, as in the really small becomes really big), the subject, which in this case would be your geckos,....would have to be 3 1/2 inches
from the front glass of the lens....anything farther than 3 1/2 inches away becomes simply close up but not macro.
With the 100mm Canon Macro, the geckos can be 6 inches
from the front glass of the lens and still get 1:1 magnication (macro). Anything farther away than 6 inches will result in simply close up images, but not macro.
So the question becomes, if you desire macro pictures...can you get the front of your lens within 3 and half inches of your geckos?
Most geckos that will be a hell no, but it really depends on what type of geckos and how tame/hand holdable/non-easily-spooked they are...