As others have suggested, it really depends on how the word is used.

When I type both words into Wordtracker, a keyword analyzing tool "big dick" would pull in 7091 searches a day, while "big cock" = 6655; "big black cock" = 1616 and "big black dick" = 1301; but "huge cock" = 7339 and "huge dick" = 2573.

You may want to download this tool to help you, I think it's still free:
http://www.goodkeywords.com/products...uggestions.php

Michael