What is the most efficient XPath expression for retri

Hello XSLT experts!

I want to retrieve the first child element. The children consist of a
<Total_Size> element followed by many <String> elements (perhaps thousands of
<String> elements).

<COFF_String_Table>
    <Total_Size>4443</Total_Size>
    <String>.eh_frame</String>
    <String>.debug_aranges</String>
    <String>.debug_info</String>
    ...
</COFF_String_Table>

Question: What is the most efficient XPath for selecting the first child
element (the <Total_Size> element)? Assume the context node is the
<COFF_String_Table> element. Which of these is the most efficient XPath
expression:

(a) child::*[1]
(b) child::Total_Size
(c) something else (what?)

/Roger