Performance-wise, there is no difference. Its been discussed over and over and over again. Googling for "join syntax oracle vs where" results in several good articles, including the one on this site referenced by Alexander.
However, be careful using a NATURAL JOIN. It will pick up on common columns like createdate or createuser or such that you normally really don't care about joining on and may cause problems. I highly recommended against NATURAL JOIN in production...just use INNER JOIN and specify the columns.
Even Tom agrees.
