i am currently using the Tamron 90 and it is a great lens as far as image quality is concerned.
However, the front element moves and can be quite irritating for carefuly composed tripod shots. For macro-type subjects, the picture composition is completely off by the time focusing is done, and the whole tripod has to be moved to recompose. Moving the tripod would of course throw the focus off, which would necessitate a re-focus, which in turn would require another tripod-move-recomposition, followed by another re-focus, followed by...
If your picture is a thin DOF flower petal swaying gently in the wind, you can recompose until the sun sets.
If tripod-style macro work is your thing, you need a non-moving front element macro lens. Else the Tamron is great, at about half the price of the Canon.
I was in this position. Decided to go with the Tamron 90mm
For portrait shots, I will use my 70-200 f2.8 IS. So the Tamron 90mm is solely for macro shots.
Read quite a few reviews, including looking at the MTF charts at photozone.de.
MTF charts might not be indicative of real world performance, but at least it gives some hints
The Tamron 90mm could possibly be slightly sharper than the canon 100mm.
Some short comings of the tamron 90mm are:
1) built quality - very plastic
2) AF is slow compared to USM of Canon 100mm. Hunts in low light
3) No FTM. - However, switching to MF is easy
4) No internal focusing
1) Possibly slightly sharper than the Canon 100mm
2) Price - think with the money saved, can get more gear
3) Weight - lighter than canon 100mm
4) Performs very well at f4 vs f5.6 for canon 100mm ( according to photozone.de)
Either way, think u won't go wrong with either of these lenses