Before you evaluate any captcha solutions you should develop a clear profile of your users. Needless to say, any captcha solution is going to make your site harder to use. If your users are older with vision difficulties deciphering a captcha can be extremely difficult if not impossible. Random characters vs real words, may impact your users. Native language may affect user's perception of what the character represents (special characters).
No recommendations here, but knowing your users and what they will tolerate and or are capable of, should have a significant impact on your decision.
