The only true way to answer that question is to shoot both from your gun. I, like you, have both a JH .660 and a JH .665 for my 21" 870 and have shot them both through my gun using 3" #5 standard long beards. I actually prefer the pattern of the .665 over the .660. The 40yd pattern looked more evenly spread with the .665, and my 20 year pattern opened up a bit more from say a softball size pattern to a volleyball size. Might help kill one if I'm a little off on a short shot.