I am looking at a nathe bit for a soft mouthed horse. She liked the bumpy happy mouth (not legal on the flat), hates the rubber mullen mouth and really likes the kk ultra taped with latex (also, obviously, not legal). I have seen the happy mouth with the smooth straight bar mouth piece but felt like it would be too rigid for her and chose not to get it. Can someone help me compare the 2 nathe choices (std vs flexible) and advise on which she is most likely to like?