In response to David's question, it's really a personal choice, although (short answer) I would suggest the 100mm macro for the increased working distance (generally, even though at 1:1 it becomes closer to the working distance of the 60mm than it does at any other time, as a result of the lens' internal focusing).
However, he is in Indonesia, and I have seen many, many shots taken with a 60mm lens (equivalent focal length between 90mm and 96mm depending on whether by Nikon or Canon) from that area, in which the species being photographed did not seem to mind in the least. The fact that the 60mm will allow you to get that much closer for 1:1 (without the use of diopters or teleconverters) will sometimes allow you to throw more light on a subject, and use a higher f-stop for more DOF. The flipside of that is in some cases, you are so close to a subject, that it is difficult to aim your strobes in the manner that you wish.
If you are used to using the 100mm lens on a FF camera, then the 60mm macro will provide that same equivalent field of view on the Canon EF-S capable cameras, albeit with a lesser minimum focus distance. Otherwise, as nap will soon find out if they do decide to go to the 5D, one either has to crop or purchase a longer focal length lens to achieve the same 'look' as one achieves with the 100mm lens on a cropped-sensor camera (that is, of an equivalent focal length lens of 160mm).
If you are going to be shooting primarily small subjects, go with the 100mm. In the worst case scenario (bad vis), you can throw on a diopter or teleconverter (or combinations of the two), and you will not only be able to bring the minimum focus distance closer, but also achieve increased magnification at any distance (compared to the previous magnification at the distance).
If you don't understand the need for the EF-S lenses out now, you have no understanding of how the sensor crops the image from the lens. The EF 16-35 f/2.8L is one of the widest full-frame zooms that I know of - and it's a heavy, expensive beast (I know, I've used it frequently). Because for a cropped sensor, the image circle projected by the lens does not have to be as large, the lens projecting the image doesn't have the same restrictions as with a full-frame camera. Optics will be lighter, smaller, cheaper, and available in wider effective focal lengths. As a result, we have the EF-S 10-22mm lens, which (with the 1.6x crop) delivers an equivalent 16-35.2mm focal length. A 10mm full frame prime lens would be incredibly expensive and heavy, a 10-22mm full frame zoom lens, unthinkable. Instead, we have the relatively light and inexpensive 10-22 WA zoom. Granted, I wish it was a bit faster, but I can live it with for now.
I hope this clears up some issues.
~Matt Segal
