Which Oly Macro: 50mm or 60mm?

I too do a load of macro and with such field application, most of the time its the environment demand that's really more a dictating factor than the actual usage demand .. You did say organic subject but that can be flora and well plants do not object to us getting up close ( usually at least ) but if your organic subject involve something like say a venomous snake, then for your own sake ( and that of the subject ) it might be better to seek out options beyond just the 50 or 60mm.

Personally with your said goal I see no real difference with the 50 and 60mm bar the fact that the 50mm only goes to half life size while the 60mm goto life size and that might or might not be the factor for you. Otherwise I think they are both good. But considering I would instead like to go for a good old Nikon 200mm /4.0 Micro-Nikkor ( the current version would do fine ) or similar from Tamron, Sigma or Canon's .. the longer focal length in this case is not for anything specific to imaging. its for the capturing the longer working distance allowed allow much more liberty in lightning and composition as well as allowing that safe margin ( for you and your subject ). One other way we used to do it in film days and still valid is to use a fine quality Tele ( like 300mm or more ) with extension tube. Do remember no matter what lens, at these macro setting ( close to or even exceed life size repro ) the Bellow Factors kicks in , so your f/2.8 aperture really is turning in light intensity more like f/5.6 or even lesss ( You can google for the complex math required for that )

Which goes back to the lens, and why I am saying a real Quality ( optically ) mid range speed ( fast would be nice but not absolutely needed ) Tele Macro would be my no.1 desirable lens right now for M4/3 ; preferably with a tripod bush that allowed the lens to be tripod locked to its nodal point. Something like a 180 or 200mm Macro that can do 1:1 and speed like f/3.4 or 4.0 .. that can easily double as the mid range " general " purpose Tele