Matter of class/instance form/function...

[Duck <duck@...>]

(I hope I get the OO terminology right.) If I want to tell a table t to be 
an instance of a class c, it seems that there are many choices. What are 
the pros and cons of each of these, for example:

 t.__index = c
 setmetatable(t,t)

or:

 setmetatable(t,{__index=c})

or, after making c the target of its own metatable's __index in either of 
the above two ways, this:

 setmetatable(t,c)


The first seems the most visibly self-documenting; the second seems the 
purest (but needs another table for every instance); the third seems to be 
a bit of both.

What does the received wisdom of the list say?