Never had an issue with either brand. They both seem to work equally well with the SIGs I own. Only thing I can really see is that the Mec-Gars have a better finish to them...maybe comes down to a bit better QC. A couple of my Check-Mate mags had a bit of rust on the springs, and one in particular had excessive build-up of the finish coating at the base of the mag (in the inside), where the mag spring floorplate is pushed in, just before sliding in the mag baseplate. I had to remove the excess coating in order to assemble the mag properly. I know it may sound a bit confusing, but I was a bit surprised...the Mec-Gars for me have always been "plug and play" with zero issues right from the start...again, which may be a simple QC issue, and not a function issue.