If the ergonomics of cameras are important, I would like to point out that the newest Canons, starting with the 7D and 5D Mark III, feel much different than their predecessors, while the newest generation of Nikons tends to be more like bricks in the hand. In the past, I preferred the feel of Nikons, but that changed with my purchase of my first Canon 7D. My latest trip to a "big box" retailer, to handle a D800, D600, 5D Marks II and III, and 7D, in one visit, confirmed this trend. (My "gold standard" for ergonomics is my Nikon F6, which, though still currently listed, dates from the the time of the ergonomically wonderful D2-series of Nikon professional digital cameras.)
Of course, this is going to vary from person to person, but I have noticed on various forums that many others, too, seem to be disappointed in the handling characteristics of the newest Nikons. Of particular note is the thumb-rest area.
Obviously, ergonomics would have to be very important to be the sole reason one would spend well over a thousand more USD to buy the 5D Mark III, rather than the Mark II.
As for a macro lens for insects, consider the excellent 180mm from Canon and the 200mm Nikkor, both
of which are well-regarded, and provide more working distance than the 100/105mm lenses. If your
passion is for an individual lens, let that drive your purchase, rather than the camera brand. One can learn to use any brand of camera, and if shooting macro with a tripod, ergonomics will not be so critical, as the tripod is holding the camera. This is why I am considering the very uncomfortable brick-like D800E as an eventual purchase, as I would use it with a tripod.
Canon 7D2/7D/5D/40D/1D2N/M3; Nikon F6/D700/FM3A/1Dx/Coolpix A; Canon 40mm 2.8 STM, 135L, 50L, 35L, 50mm 1.8 I, 100mm 2.8L Macro, 10-22mm EF-S, 400mm 5.6L, 16-35mm 2.8L II; Nikkor 50mm 1.2 AI-S, 50mm 1.4G, 50mm 1.8D, 16mm 2.8D Fisheye, 180mm 2.8D, 100-300mm 5.6 AI-S, 45mm 2.8 AI-P, 14-24mm 2.8G, 24-70mm 2.8G, Micro-Nikkor 60/2.8G; Voigtlander 90mm f/3.5 SL II, Zeiss ZF.2 2/135 APO