We provide several captcha options that vary in ease of use vs protection.

I would base any decision only on real feedback from real users. I do not know of any success rate statistics based on typical users. While inconvenient I think most real people can solve any of the captchas, though with Re-Capthca some things may be easier to solve than others within the same Captcha because they are using OCR to transcribe books, the user can click to get a new challenge if they can't solve it.

Success rate based only our own or my own site traffic would be meaningless and more likely a high failure rate just indicates a lot of bot traffic.

Best,

Joe