On the brush, the key factors are ability to hold water/lather and the ability to whip up a lather. For the first criterion, a brush with a good "bushiness" means lots of internal volume for holding water and lather. The bristles should be "poofy". For the second criterion, the hairs should be soft and supple (at least with a good soaking in hot water). Hairs that are brittle and have little or no flex to them will not work so well. You want each bristle to be like a little whip whipping little air bubbles into the lather. What you don't want is the entire brush being like a stick stirring the soap and water around.