In a nutshell:

memcache is older, less mantained.

memcached is newer, developed by Digg people and generally preferred. Also it seems it is more configurable and has more options.

Complete explanation and opinions: http://serverfault.com/questions/63383/memcache-vs-memcached

Client comparision: http://code.google.com/p/memcached/wiki/PHPClientComparison

PECL pages:
http://pecl.php.net/package/memcache
http://pecl.php.net/package/memcached