I would opt for the longer focal length. It may be a bit heavier, but you'll get more working distance with the same magnification ratio. It also will take in less background, making it easier to separate your subject from the background. As far as the auto focus - use it for normal shooting (portraits, etc.), but for macro use, set the focus manually depending on the magnification you want, and then move your body (or, better yet, tripod w/focussing rail) in and out until the subject is focused. When shooting tight macro shots, the auto focus is changing the magnification rate more than the focus plane. if you want patience - get out before the dew dries, and the bugs won't be moving at all! Just a couple of thoughts