My experience with macro is with unmoving small archaeological objects, down to about the size of a small fingernail. I've used a ring flash, never the twin flash, but my best shots have always come with natural light and reflectors. I've got some of those small collapsible ring reflectors in silver, gold and white, that are about the size of a dinner plate, and I've found that you can get much subtler effects and (I think) better color transitions than with flash; I've always thought that the flash was just too close, and too directional, but then, I've never used the twin flash arrangement. With the reflectors, you can focus and then look directly at the object (instead of through the viewfinder) and manipulate the light until it's exactly like you want it, before firing the camera.
My biggest problem was that I used to do this outdoors in the desert in the Middle East and I'd sweat so badly that sometimes I couldn't see...
JC