Definately the 100mm macro. Better build. Non-EFS so if/when u upgrade to FF, you can still use it + normal extension tubes/TCs will work if you want more magnification. The longer working distance makes it easier to shoot the more jumpy insects.
Thanks Dream Merchant, daohuay & Denosha for your opinions!
My main intention is shooting macros, but I thought it will be helpful to have that extra distance too. I went to the bird park but my 17~85mm lens is limiting that "3D" stand-out effect on those birds that are a few meters away. Will this 100mm helps? I read from the previews that both are great lenses, deliver sharp images (and both are slow in focusing under dim light environment) but none really compares the two.
I'm ok with carrying the tripod, and it's plus that the non EF-S lenses can mount to a wider range of Canon DSLRs.
Lastly, I heard that the lenses price is cheaper in Hong Kong. Any recommended shop, retailers? If there is already a thread on this topic, do point me to it.