It doesn't have to do with OC' capabilities. Just more cache that really helps in some applications. I would go with higher cpu speed if I was to choose from the two, but:
....wait till 22 april. E6420 will match E6400 new price and same goes with E6320 and E6300.
http://www.fudzilla.com/index.php?option=com_content&task=view&id=596&Itemid=1
Ἢ τὰν ἢ ἐπὶ τᾶς - Μολὼν λαβέ
Today is a gift. That's why it's called the present.