I'd go the Canon over the Tamron any day of the week. The slow autofocus alone is enough to turn me off the Tamron - as Whatsreal mentioned, there's nothing more frustrating than poor autofocus - the keeper rate is significantly lowered. Fast focus is pretty important with macro photography, especially with insects etc.


I'd trade one stop of light for quick focus any day. In saying that - it is nice to have the larger aperture when you need it but looking at the kind of shooting you're into I think f/4.0 will cover your needs. Have a look at FastGlass's photo above of the midget Kangaroo, it was taken at f/6.3 and the background blur is more than acceptable.


I agree that you should investigate the Kenko extension tubes as well. I own them and they work very well and are a cheaper alternative to a macro lens. That should give you some good flexibility anyhow.


Enjoy.