I think you'll be happy with either of the Canon 100mm macros. They are both excellent lenses. I've been using the USM (non-L/IS) for a number of years and it's very handy focal length on both crop and full frame.
Like any macro lens, focus is a little slower than equivalant focal length, non-macro lenses. A macro just has to move the focus group a whole heck of a lot farther, to be able to focus all the way from infinity to 1:1. But the USM helps, and the lens has a focus limiter switch you can use, that make it reasonably usable for non-macro purposes too.
You asked about the 100/2... and besides the obvious difference in max apertures and the macro focusing capabilities, the macro lens is a "flat field" design and the 100/2 is not. A flat field design is optimized for close focusing, to produce edge to edge sharpness at close distances. You could pop some extension tubes behind the 100/2 and get it to focus a whole lot closer, but the corners of your images would be softer and likely there would be some vignetting too.
One nice thing about the Canon 100mm macro lenses, they can optionally be fitted with a tripod mounting ring. It's not included standard, but can be a very handy accessory with a macro lens. The Canon 50mm and 60mm macros cannot be fitted with a t'pod ring. AFAIK, most of the third party lenses in the 90mm to 105mm range also can't be fitted with one. Note that the Canon t'pod ring is pretty pricey... there are lower priced third party clones that seem fine and cost a lot less, on eBay and elsewhere.
The Internal Focus design of the Canon 100mm macros makes them a fairly large lens, but they don't change length when focused, the way some macro lenses do (I'm not certain, but the original Canon 100mm non-USM macro might not have been Internal Focus design... both the current USM and L/IS are).
The lens hood for the Canon 100/2.8 USM is huge, but recommended. The front element is far forward and exposed, so the hood is helpful for protection both from oblique light and from bumps. If the hood gets in the way, it can always be removed temporarily. I've also got a second, smaller hood for mine (screw-in 58mm, I think it's from a Tamron telephoto lens). I use that with certain flashes, when the Canon hood gets in the way. There are third party clones of the Canon hood that are considerably less expensive.
Another reason I like the 100/2.8 USM is that it shares filter size with many other Canon lenses: 58mm. The newer 100L/IS uses 67mm, which fewer other Canon lenses use.
P.S. Even if you get the 100mm macro, you still might want a set of macro extension tubes eventually. I've used them together at times, for greater than 1:1 magnification. I think you can get close to 2:1 or twice life size, using the Kenko set with the 100mm.
