I own both the 180 and 100 mm macro lenses.
For any nature photography, definitely go for the 180 mm macro. As others have noted, depth of field depends on subject magnification rather than focal length, so at 1:1 you'll get an identical depth of field whether you're using a 50 mm, 100 mm or 180 mm lens. What will change is your background coverage. It's much, much harder to exclude distracting out-of-focus background stuff when using a 50 mm macro. The far narrower angle of view provided by a 180 mm lens makes it a lot easier to avoid ugly intrusive background garbage. The greater working distance at 1:1 with the 180 mm macro makes it far easier to use for photographing flowers or bugs out in the real world, especially with a full frame D-SLR. With a smaller sensor APS-C camera, you get even more working distance. You can never have too much. Nothing's quite as maddening as maneuvering to photograph a backlit spiderweb loaded with dewdrops, only to bump the plant it's suspended on with the tripod leg because your working distance is so tight. Splash. No more photograph.
The newer 100 mm macro is great as an all-around lens, as it's scary sharp and terrific for portraits, where the IS function is invaluable. It's light and compact enough to bring along on hikes even if you don't plan on shooting macro images. The 180 is a fabulous lens optically, but it's much bigger and much heavier.