Sequence of bytes - userdata or string?

[Asim <asim.r.p@...>]

Hi All

I want to write a C function that returns a sequence of bytes.  This 
function should be called from Lua.  There are two options for the type of 
return value in Lua - userdata and string.
The Lua function that will call this C function wants to access individual 
bytes of the returned sequence.  If string is chosen as the type of return 
value from C, then the bytes can be accessed as string.sub(ret_str, i, i) 
where i ranges from 1 to length of ret_str.  string.byte(ret_str, i) is 
also an option.
If userdata is chosen as the type of return value from C, then a metatable 
can be created by the C function before returning the userdata object. 
The metatable should allow indexed access to individual bytes in the 
userdata object (Is it possible to write such a metatable?).  This would 
allow ret_val[i] expression to access individual bytes in Lua.
How to evaluate (and hence compare) the cost of each of the above 
approaches?
Asim