Back when I was deciding to buy my first macro lens, my two candidates were the EF 100mm (the 100L wasn't around back then) and the Sigma 150 (No OS version was available).
I ended up buying the EF 100mm (which still blows me away with its quality) and the final decision was made on price. The EF 100mm was 50% cheaper than the Sigma.
PS With a 12mm ext tube you get 0.30x and 0.37x with 25mm (MFD on the 300 f/4L)



