1/2 inch shank is always better. The forces at play with a router are terrifying enough as it is. A beefier bit cuts the risk of flying shrapnel. You also want to use the correct router speed. That is often overlooked. You want to keep the rim speed under control. A 2 inch diameter bit should generally be under 18,000 rpms. A 3 inch bit should be under 14,000 rpms. The reduced rpms help keep the rim speed under control. A half inch diameter bit at 20,000+ rpms will have a rim speed near 40mph. A 3 inch bit at the same rpms will have a rim speed over 200 mph. A .25 inch shank is more prone to failure at those high speeds when you crash a hunk of wood into it.