I've used both. If there is a difference optically, it's completely insignificant in practice. They are both extremely good. I've found that the IS doesn't help as much for macro as I had hoped, but it is helpful if you use the lens for non-macro distances (for e.g. candid portraits). For macro, a flash is much more useful. You could use the price difference to get e.g. a Sigma EM-140 DG Macro Flash.
Comparing the EF 100/2.8L to the EF 135/2.0L for portraits, I would again say that the difference in sharpness (and bokeh) is insignificant (they are both excellent), the most important difference is instead the larger aperture of the 135L and the IS of the 100L.