When it comes to macro lenses, there really isn't a lot to concern yourself when it comes to Sigma/Tamron/Nikon/Canon. Optically, all are going to be extremely good as any little flaw will be more than apparent. The Sigma 105 and 150mms are some of the most popular macro lenses. Very comparable to the N/C counter parts at much lower price points.
How "fast" do you want/need?...aka f2.8 vs f3.5mm
Well that's a personal preference that no one can really answer for you and for those of us that try...we are "biased" and predisposed based on "our" needs.
I'll admit I'm a fast glass junky. Which means if Im eyeing a lens purchase and I have a choice of getting A) thats f2.8 and B) that's f4 I will usually (not always) favor/want the faster glass and save for it if need be.
While you have options if SS is the concern (bump ISO) there are limits. I do a ton of low light and often stop action in low light so thats important. Then to I use filters a lot so again the faster the lens when I add a filter Im left with a higher SS all else equal. I also really enjoy the options for DoF and artistic choices faster glass affords. While you can replicate this in post there are limits. Basically when eyeing the Canon 24-105 f4 L and the Canon 24-70 f2.8 L I opted for the faster f2.8 because I felt ultimately it would give me more options and more options/choice the more power I have. However as I said, I am heavily biased both in terms personal preference and need.
"but"
With Macros, unless you are using for things other than Macro photography (and you will) having glass that fast is almost immaterial because you will almost always be at f11 and lower.
So how often would you open a macro lens to f2.8?
Not very.If Ive shot 10,000 images with my macro Im willing to bet less than 50 were shot at f4 or wider.