I hear you. I've stuck with the Canon 180mm Macro despite people saying the Sigma counterpart is just as good and cheaper. But I got a deal, so it was just as well. The 105mm should be just fine, don't go any shorter though or the background blur won't be as pleasant. Here's an example:
Which is why I suggested the 180, but 100ish is still great. For good technique, you wouldn't want to aim down towards the ground anyway, but rather have more distance parallel to the ground, so the background is a lot farther away.
Here's the main article, which explains why this happens, as it's not a magical focal length thing, but rather a side effects of the distance you're forced to use when you want the same framing.
http://the-digital-picture.com/Reviews/Canon-EF-180mm-f-3.5-L-USM-Macro-Lens-Review.aspx
And I picked up the FS100 as well. Finally cashed in my 30,000 credit card points.