What is the difference between the two? Does the purely numeric and easily readable one work as well as the cryptic one?

No is the short answer. The evil people just plugged them into OCR packages, hence the escalation in obscurity.

See Another way to get around automated bots and Image Verification Program for some previous threads on image based captchas. Personally I find them annoying, not particularly effective and a complete pain on the accessibility front.

In reply to Re: [OT] Some questions on captcha by adrianh
in thread [OT] Some questions on captcha by kiat

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