I am looking at Macro rings instead of a flash unit (I have an old flash unit). I am torn between the Sigma EM-140 DG Macro Flash (I'm a huge Sigma fan, can't afford legendary 'L' glass, but Sigma's macro lenses are fantastic if you get a good one - QC) and the Canon EM-140 DG Macro Flash. The Sigma seems to have more adapters for more lenses.
I've tried the Sigma and own the Canon. Both are quality flashes with ratio capability (a must).
The only draw back I could find with the Sigma was that you had to use an adapter ring. The Canon fits directly on to my 100mm and MP-E. It will also fit directly to the EF-S 60mm. If I remember correctly, adapter rings are available to use the MR-14EX with the 180mm and I'm reasonably sure you have to get a ring for non-Canon lenses.
Seeing that you're using a Sigma 105mm and will likely need a mount ring, I'd be inclined to save a little and go with the Sigma Flash.
Two thoughts, regardless of which flash you settle on. First, learn to use the power ratio to help add some depth to your shots and avoid washing them out. As close as you are, you won't need much power to begin with.
Second, experiment. I've gotten to where I power down one tube completely, the other is powered down to 1/32-1/64th and then rotate that flash around the lens barrel to get the better lighting angle depending on my orientation to the subject.
