There are (or were) three 9mm Sigma models. The larger two are really copies of the Glock. The smaller one called the Sigma 9M was an attempt to further reduce the size and cost. The 9M has a zinc alloy slide with a limited life and has been discontinued.
The larger two are better, but are not the quality of Glocks. They had problems at various times in their production, but some are quite good. If you have a chance to thoroughly test the Sigma and have a return guarantee, it might not be a bad purchase. S&W has good service and the last I heard they were still providing free repairs unless the gun was abused somehow, although shipping charges to get the gun to S&W are your reponsibility.
S&W was sued for patent infringement by Glock concerning a minor detail of the Glock that they copied too closely. I believe the suit was settled out of court.
Hope this helps.
Alan W.