Been a few posts about macro lenses of late. I have the Canon 100, Sigma 105, and the Canon MP-E 65. I would not recommend the MP-E for a beginner, it will just frustrate you. It frustrates me
Around the 100mm focal length is great for a few reasons. It gives you fair working distance (ie the distance from front of lens to subject) for what you spend on the lens and if you really get into macro and decide to get extension tubes, 68mm of extension tubes will give you up to 2:1 magnification with this focal length. And longer and you won't get that mag, any shorter and you won't be able to focus or it will be ridiculously close to the front of lens.
Most macro lenses are tack sharp by nature, and the ones recommended are no exceptions. I haven't used the Tamron 90 myself, but close friends have and they do nothing but rave about it and I'd have no hesitation passing on their recommendations either. I recall one telling me he thought the Tamron had nicer bokeh/diffraction properties than the Canon and Sigmas - something to do with the aperture blades, can't recall if it was because there were more or they were slightly curved. In any case, the difference will be negligible to you or I - he was really pedantic and pixel-peeping.
If it was me in your position, I'd get the Tamron 90. Purely because it's a smidge cheaper than the Sigma 105 and much cheaper than the Canon 100 (both versions).
Some tips for you when you do get it and want to start shooting.
Next lesson will be macro lighting, but that's a whole new thread
