I do not own any of these three, but do have a few thoughts, with the disclaimer that I do not regard myself as any kind of authority on macro lenses, though I find macro very important as one part of my crime scene photography. With a good macro/telephoto lens, I may be able to shoot a whole scene without changing lenses.
First, I do not use filters on macro lenses, so the uncommon filter sizes of my Canon 100mm Macro 2.8L and our Tokina 100mm 2.8 are non-issues.
Second, be sure to consider your working distance for 1:1 in relation to your subject. Living things may not tolerate a lens being too close to them. The listed distance in specifications, unless otherwise stated, is usually from subject to sensor.
I do remember that one of my local instructors compared a Sigma macro lens with a Canon macro lens, at the dealer, and found the individual Canon to have more distortion than the individual Sigma, so he purchased the Sigma. Keep in mind this could be sample variation, and does not mean all Sigmas have less distortion than all Canons! The lesson here is to try the individual lenses, if feasible, before purchase.
Consider whether you want your macro lens to serve double duty, as a telephoto. That is why I bought my 100L, because I wanted a fairly fast weather/dust-resistant lens with Image Stabilization in the 100mm range, with a night-time aerial surveillance assignment speeding my decision to buy when I did. (A 70-200mm 2.8 with IS was beyond my budget at the time.) The beautiful bokeh of the 100L adds to its usefulness!
While on the subject of bokeh, try taking a look at the Tokina 100mm, which has nice bokeh. I like the push-pull AF-MF clutch, though now that I have my 100L, the Tokina sees little use.
Canon 7D/5D/40D/1D2N; Nikon F6, D700, FM3A, & Coolpix A; Canon 40mm 2.8 STM, 135L, 50L, 35L, pre-II 50mm 1.8, 100mm 2.8L Macro, 10-22mm EF-S, 28-135 EF, 400mm 5.6L; Nikkor 50mm 1.2 AI-S, 50mm 1.4G, 50mm 1.8D, 16mm 2.8D Fisheye, 180mm 2.8D, 100-300mm 5.6 AI-S; Tokina 17mm & 100mm 2.8 Macro
Last edited by RexGig on Sun Jan 08, 2012 10:23 am, edited 1 time in total.