Based on the cases I've researched, it seems to me that sex offenders are mostly opportunistic. If it is too risky to target a child, they might take an opportunity to target a vulnerable woman, and vice versa.
The FBI states "Serial sex offenders frequently assault both strangers and acquaintances and often exhibit intraserial variations in victim relationship, age, and even gender."
From the BJA: "Sexual predators are more opportunistic than we thought, based on the crossover we discovered. By that we mean that they often mix their victims – the same predator who assaults a teenaged girl may also assault someone elderly, or across racial or even gender lines. Exploiting victims' vulnerabilities became more of a common thread than other variables."
Richard Davis attempted to abduct an 8-year-old approximately 4 months before Barbara disappeared. He was questioned by the police and it probably scared him. It is possible that this event made him believe it was too risky to abduct a child in broad daylight. In contrast, it was much less risky to abduct Barbara, who was a woman walking home alone at night. It's also interesting to note that the abduction attempt and the disappearance of Barbara occurred at the same intersection.
In regards to the confession by Frederick Bush, he subsequently denied it and never confessed to the police. He merely made a passing comment to one of his co-conspirators, which I think was an attempt to sound tough and scare them. It's not clear what his motive to abduct and kill Barbara would be, as his other murder was vastly different.
I'm not completely sold on either suspect. However, I wouldn't rule Richard Davis out considering it was stated he had a history of coercing both girls and women, he lived in Bath at the time, the abduction attempt was at the same intersection where Barbara disappeared, etc. MOO.