What are practical differences between `REPLACE` and `INSERT … ON DUPLICATE KEY UPDATE` in MySQL?

Question

What I need is to set the values of all the fields of a record with a particular key (the key is composite actually), inserting the record if there is no record with such a

Answer 1

Replace seems that it does two operations in the case that the key already exists. Perhaps that implies there is a speed difference between the two?

(INSERT)one update vs one delete + one insert(REPLACE)

EDIT: My implication that replace might be slower is actually completely wrong. Well, according to this blog post anyway... http://www.tokutek.com/2010/07/why-insert-on-duplicate-key-update-may-be-slow-by-incurring-disk-seeks/