I have Canon 100 mm f2.8 macro, and it's an awesome lens. I also have extension tubes. That combination is extremely powerful and can take some very nice magnifications. The main difference (apart from the price tag) between 100 mm and its wider cousins is the distance from the lens that will give you the 1:1 magnification (all Canon Macro lenses can do it). With wider lenses you need to come closer to the subject.
That being said, I'm still struggling with decent photos of ants. My problem is that they don't like light, and you need light to take photos. So ants walk around try to hide etc. It is not easy to keep track of them when your view field is so narrow and depth of focus paper thin, as it is with this setup. If ant is in a test tube or other container, you get distortions from the plastic/glass, or there is dirt in the way.
Also, when you start playing with high magnifications, you need lots of light, especially if you close down the iris to get some depth of focus. Decent lighting is also a good way to reduce the problems with glare from test tubes and glass, but this is not something I can help with much since I too am still figuring this one out. I loaned my speedlight so I'm struggling a bit with continuous lights. Theory say that a light box should be helpful, but I can't find mine.
Long story short, if you're in the market for a Canon marcro lens, I'd advise you buy the longest one you can afford. For extension tubes, do not cheap out and buy the dumb ones. Get the ones with electrical connectors for autofocus and, more importantly, iris control, they are certainly worth the extra $5.