Iggyotis, honestly if you can live with 100mm, forget the 135mm f/2 and go with the 100mm f/2.8 Macro IS. It is not only a much better low light lens than the 135mm, but it's better all around. It's noticeably sharper than 135 at f/2.8, with way less vignetting. The IS DOES matter, because to get a sharp pic with the 135mm you need at least 1/135sec shutter, whereas with the 100mm Macro, because of the hybrid IS, you can handhold and get a sharp pic even at 1/25 sec, so you can more than make up the one stop f difference between the 2 lenses.
Secondly, the 135 does not look that good wide open, I personally wouldn't use it at f/2 - it's a tad too soft for my taste. Even when you stop it down to 2.8, the 100mm macro makes it look silly in comparison.
Thirdly, if you happen to need to shoot something that's closer than the minimum focusing distance that the 135mm allows (I think it's a minimum of 1m or so, maybe more) you'll be happy you have the 100mm macro on, because you can shoot from a foot away, if not closer.
Last year I was trying to decide between these 2 lenses, tried both of them, analyzed test shots on a pixel level, and I hate to say it but 135mm did not live up to the hype. It looks impressive mounted on the camera, but way too limiting, and ultimately not as good.