The main difference is sorting accuracy (when comparing characters in the language) and performance. The only special one is utf8_bin which is for comparing characters in binary format.
utf8_general_ci
is somewhat faster than utf8_unicode_ci
, but less accurate (for sorting). The specific language utf8 encoding (such as utf8_swedish_ci
) contain additional language rules that make them the most accurate to sort for those languages. Most of the time I use utf8_unicode_ci
(I prefer accuracy to small performance improvements), unless I have a good reason to prefer a specific language.
You can read more on specific unicode character sets on the MySQL manual – http://dev.mysql.com/doc/refman/5.0/en/charset-unicode-sets.html