I'd echo what Danny said - you cant go wrong with a prime macro lens (the 100mm range at least) - they all double as a "portrait" lens.  If you have a 50mm lens anyway i'd do the 100 macro (if not get the 50mm 1.8 canon - its rude not to at the price ).

I have the 105mm sigma, basically i would only go for the canon if i decided i wanted to spend that little bit extra for the USM, i'd imagine the macro David uses is comparable to the sigma in optical quality... never picked one up really... but I've personally never had much of a problem with the focusing of the Sigma... i managed this shot on auto focus with the 105mm:

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography