I read the reviews about the two lenses, it appears that the front elements of the tamron lens will extend while focusing, but the canon one affords an internal focusing. If the macro shots that you intend to take involves insects, in this case, I'd think the canon one is better. But I don't really know since I have never actually seen the two lenses before.
Both lenses are good on their own but if you can wait out and save a bit more, the Canon will probably serve you better in the long run. Better optics and slightly more accurate AF... comparatively speaking.
I was just thinking about the same thing a couple of weeks back. I ended up getting the canon 100mm. I got a friend who was heading to HK to get it. It was only 780 SGD there with International Warranty. Can consider that path. so, only need to save 200 more.