The Canon lens is made specifically for the Canon SLR, while the Tamron is a 3rd party lens made to fit most SLR's with adapter mounts.

This means the Canon is faster to focus, quieter and has an internal focus. While the Tamron comes quite close to these factors, it has an external focus. When the Canon focuses, the lens shape and length remains the same. When to Tamron focuses, the length will extend longer. In most cases, this is not a significant issue. If you want to use a top-down box to shoot pictures from the top of the aquarium, then you put the camera lens into the box and put the box in the water. As the camera focuses, the external focus lens will lengthen and may hit the bottom of the box, whereas the internal focus lens will remain the same length regardless of the items in focus.

As I stated previously, if you can do it, get the Canon Macro lens and be satisfied that you got the very best. If your budget doesn't allow for the Canon, get the Tamron, knowing that it is as sharp as the Canon, but that you may have to adapt to some small differences, like external focusing causing the lens to extend longer as it focuses on closer objects.

HERE is an excellent comparison of both the Canon and Tamron Macro lenses