I love CAPTCHAs. Why? They're interesting. And, often interesting to behold.
But do more challenging CAPTCHAs correlate with better sites? I would guess yes, as if someone really cares about their site, they'd change the default settings in their captcha plug-in or use something they've found or created themselves.
So is it true? I do not have nearly enough knowledge about which captchas are for which specific target types and engines, and also which targets statistically tend to have better trust, rank, higher SERPs, etc.