A circumcised penis has had the foreskin -- which is attached to all penises at birth -- removed. This is often done for religious reasons, and sometimes with the idea it's for health reasns, though medical associations have long since refuted that removal of the foreskin is needed or even desired. To know where the foreskin is/was, see: Man's Best Friend: Male Sexual Anatomy.

The foreskin, similarly to the female clitoris, though with far less nerve endings, is rich with nerves for sensation, and provides a penis with extra friction and some lubrication during sexual activities. Men without a foreskin often experience less -- or different, depending on how you look at it -- sexual sensation, require more friction during sex, and with female sex partners during intercourse, more lubricant is often required.

Per whether or not you should have a preference, think about it this way: how would or do you feel about someone choosing you as a partner based on if you have small breasts or large ones? How respectful is that to you as a person, and how much are preferences like that about treating people LIKE people, not objects?

BOTH forms of penises are functional, and BOTH forms of penises can provide sexual and/or aesthetic pleasure to men and women.