As a first macro lens I'd recommend you get something in the 90mm to 105mm focal length range, especially since you will be using it on a crop sensor camera. This will give you reasonably good working distance from shy/stinging/biting subjects, better than shorter focal length lenses. It's also more easily steadied than longer focal lengths, which compound the problem because they render shallower depth of field, requiring you to use smaller apertures and slower shutter speeds.
Next, decide if it's worth spending the extra for stabilization. At higher magnifications, stabilization is of pretty limited help. The Hybrid-IS Canon created specially for the 100L is probably the best available, but even with that you likely would only see about one stop's worth of assistance at full 1:1 magnification. It will be more effective at lower magnifications, until it gives three or four stop's worth at non-macro distances. I wouldn't expect much help at all, from other than Canon's IS... Nikon had to admit that the VR on their 105mm (the first macro lens to use stabilization) was of almost no help at 1:1. I doubt VC or OS from third party manufacturers would be any better. However, for non-macro shooting with the lens, you might like having stabilization (I love it on several lenses... just don't need it on my macro lenses).
Besides, if using the lens primarily for macro, a tripod or at least a monopod is an important accessory. Sure, some handheld shooting is possible (flash can help with that), but a lot of macro work is done using some sort of support.
Here's where the two Canon 100mm lenses are especially outstanding. They are the only macro lenses in the 90mm to 105mm range that can optionally be fitted with tripod mounting rings. The 100mm f2.8 USM (non-L/IS) uses Tripod Ring B (b), while the 100L IS USM uses Tripod Ring D. Personally I consider these more useful than stabilization, allowing for quick changes from horizontal/landscape format to vertical/portrait. And, the help balance the lens better than mounting lenses without a tripod ring. However, these rings do add cost ($140 for the 100/2.8 USM and $172 for the 100L IS, last time I looked... there are cheaper third party tripod rings, usually about $50, but I haven't used them).
Other key features of the two Canon 100mm:
- Both use USM focus drive, which helps make them reasonably fast.
- Both have Focus Limiters, which also can help with focus speed. The 100L's is slightly more sophisticated, with three zones, while the 100/2.8 USM offers two zones.
- Both are "IF" or Internal Focusing, which means they don't increase in length when focused closer. This makes them bigger to start, and actually causes the true focal length to change when focused closer (you don't notice this in real world use), but it makes for better balancing and limits loss of working distance to the lens extension.
- Both are well built and sealed. The 100L is, after all, an L. But the non-L 100/2.8 USM actually is identical in design and build quality to the Canon 180/3.5L... so it's certainly "L-like", too.
- Both have top image quality. I have never before seen claims the Sigma 105mm is sharper... but in fairness all macro lenses are pretty darned sharp. The Canon are both "Flat Field" design, which basically means they are optimized for edge-to-edge sharpness at close focusing distances, but so are most macro lenses.
- The 100L comes with a lens hood and use a 67mm filter.
- The 100/2.8 USM uses a 58mm filter, but it's hood is sold separately for about $35.
- Both are designed to accommodate the Canon MT-24EX Macro Twinlite, MR-14EX and MR-14EX II Macro Ringlites. The 100L requires an Macrolite 67 adapter to fit these specialty flashes, which costs about $32. There is no adapter required to clip these flash units onto the 100/2.8 USM lens.
It's hard to beat the Canon 100mm Macro lenses. They cost a little more, but are among the most full featured. None of the third party lenses have all the above features or offer anything the Canon lenses don't.
As to flashes, I would not recommend a ring light, Canon or any other brand, for use with 1:1 or even slightly higher magnification. To me, they just look too "clinical", with just too flat lighting for my tastes. I do like and use one on an ultra-high magnification lens (Canon MP-E 65mm).
The Canon MT-24EX Macro Twinlite is excellent, but rather pricey. You can accomplish a lot with a standard Speedlite. Just get an off-camera shoe cord to use with it (about $60 for a Canon cord, half that for third party), and either hand hold or mount it on some sort of flexible flash bracket. Because a big flash might be too powerful up close, I put a couple layers of white gauze bandage over the head, held in place with a rubber band, to diffuse and reduce the flash's output. This makes it act like a giant softbox, with very small macro subjects.
These are: MT-24EX on EF 100/2.8 USM lens (left)... MR-14EX on MP-E 65mm lens (center)... and a 580EX II with off-camera cord, 100/2.8 lens(right)
This was shot with a single standard Speedlite handheld and on off-camera cord, diffused as shown above...