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Most macro lenses will go to 1:1 as a maximum magnification, but will also allow focus to infinity, thus allowing you to photograph subjects which are larger than small insects, or to take a useable shot from further away.
As a Canon user you have the option of a 5:1 macro lens, but this will not allow infinity focus so it is much more difficult to use.
The easiest way to get a macro lens to 2:1 magnification is to use a 2x teleconverter, using extension tubes is impractical, as you need to add the same as the focal length to add one unit of magnification, in other words 100mm with a 100mm lens. Bellows used to be a method of moving the lens from the film/sensor plane, but there don't appear to be any which will allow the camera to communicate with the lens and as a result it is not possible to adjust the aperture from wide open, which is usually very large on a 50mm prime.
At these kinds of magnification light is at a premium, an indicated aperture of f/8 is a real aperture of f/16 at 1:1 but at 2:1 it becomes f/24 so a flash is a must. Because the point of focus focus becomes closer at higher magnification a fing flash is not a good choice, and you should consider a true macro flash where the flash heads have an adjustable angle to account for this.
Although the Sigma 105mm Macro is optically very good the extending focus is a major minus especially when using a macro flash, as the motor struggles to extend the lens with the extra weight. The Canon on the other hand is internally focussed and it's very fast to find it. If the cost of the two lenses was further apart I'd say that the Sigma was a good choice, but there's only about £70 between them, and that makes the Canon a no brainer.
One cheap way to acheive something like macro is to buy a set of close up filters which screw on your existing lenses and work quite well.
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