Best way to use INNER JOIN?

Using Oracle 10g

I've always used this form of inner joins -

  1.  select BOOKSHELF.Title from BOOKSHELF, BOOK_ORDER where 
BOOKSHELF.Title = BOOK_ORDER.Title;

A colleague tells me that I should be using this form (but can't tell me 
why?) -

  2.  select Title from BOOKSHELF inner join BOOK_ORDER using (Title);

There is another way I've seen (but don't like) -

  3.  select Title from BOOKSHELF natural inner join BOOK_ORDER;

So, is there any difference between methods 1 and 2? if not is it safe to 
continue using method 1 then?

thanks in advance

harry