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Sigma has image stabilization, right? I don't know if that matters - my Nikon macro is a 105 f/2.8 - no stabilization.

Not this one, its a newish Lens, and they went for image quality over Stabilization, but my A77iii has IBIS which would help,
that said thats a part of the reason why I am considering saying for the sony 90MM which does have image stabilization

Good luck with your choice. I like to shoot flowers and they are always blowing around - I'm not sure if stabilization helps or not. When I attach the lens to my Olympus em-5 I get the advantage of IBIS but I don't know how much it helps really.

thanks and thats what I envisage using it for often as well, which does make me think the extra money for the Sony may be worth it, so I am bouncing back and fourth at the moment LOL