Previous posts have about covered it. With the smaller 5/16" (8mm) you will have to chisel more of them apart (maybe most of them) which will usually distort and tear the metal more. The 3/8" (10mm) results in a larger hole, but is more likely to get enough of the weld cut. I have had the most success with the drill bit style of cutters. The 'toothed' ones frequently break the teeth due to the often irregular weld surface. It is very difficult to hit (and to stay in) the center of every weld. I am currently using Irwin's Turbomax drill bits to drill out spot welds.
www.irwin.com/tools/drill-bits/turbomax-high-speed-steel-straight-shank-jobber-length-drill-bits