Most everybody I fly with has tried the 19 strand and gone back to 7 strand for that exact reason....except Howard, I think he usually uses the 3 strand soldered stuff from PAW. Not sure about Paul's preferences, since he moved to Venison Park, but suspect he's using either the 3 strand or 7 strand. I've never heard or read of 21 strand, tho.
I got bulk 7-strand from RSM...nice stuff. If you order from McMaster-Carr, be sure you get stainless steel, and whoever you get it from, make sure it isn't that bronze colored stuff. The shiny silver is much easier to see on any surface, be it grass, concrete or asphalt. At McMaster-Carr, I think you can choose from several different stainless alloys, and I have no idea which to get.

Steve