Macro lens for bugs: 90/100 vs 150/180
100mm focal length is a good compromise. The MFD is usually around 15cm, good enough for most insects. It matters more how you approach your subject than the extra 10cm you would get with 180mm lenses. I use both 100 and 180mm macros and excepting for some very skittish subjects, or when I cannot get close to the subject, 100mm is perfectly good. However, there are other differences - a longer lens will give you a smoother bokeh whereas a shorter lens is easier to handhold even with a TC.
Take a look at the Weekly Macro/Close-up thread and see what lens the posters used for their shots.