For outdoor shots of bugs, flowers and plants, the subject is often inconveniently situated at ground level or halfway up a tree, places where it is difficult to set the tripod close enough for the shot. In that case I'd go for the 200mm micro for the greater working distance and magnification. The 200mm micro also has a reputation for superb IQ, probably the best of the micro-Nikkors (although it would have to be pretty good to beat my old AIS 105/4 micro...)
For indoor work and situations where the environment is more controlled and it is easier to set the tripod within reach of the subject, the zoom factor of the 70-180 seems pretty attractive. I'd regard the 70-180 like a flexible 100mm macro. The 200mm may be too long for indoor work.