If you're looking solely for image quality, and you're using a tripod for your macro work (as most quality macro shooters do), then I would forget about the 100-105 mm range altogether and go with 180-200mm macro lenses.
For starters, you get more distance between you and the subject, which means your background bokeh will be much smoother and creamier than a 100mm can possibly deliver.
Regaring the IS (or VR) of some of the newer 100/105 mm macro lenses, these are really for hand-held fieldwork, involving moving subjects like insects and such, where lighter weight (IS and flash use) can really come in handy.
But if you're taking your time composing artistic shots of flowers, which pretty much just sit there (as do some butterflies/insects, etc.), and if you're using a tripod, then you have no need for IS at all, and the heavier weight of a 180-200mm won't matter.
Finally (regarding flash), I certainly haven't seen a flash shot that compares to the subtle pastel-like colors of the best macro shots taken in optimal natural light. But that is just my opinion. Good luck.
I couldn't agree more. IS/VR can actually decrease the quality of your image if you shoot with a tripod as I do. Maybe I should really be considering a 150-200mm lens, and then later pick up something in the 70-100mm range with IS/VR for those rare occasions that I don't have my tripod on me.