I was in this position. Decided to go with the Tamron 90mm
Reason being,
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
Advantages:
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