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The second method adds a HD access to read an included file.
On the other hand, the second method doesn't have to parse both blocks of code - just the one triggered by the if test.
However the differences are not significant. Efficiency is rarely an issue.
It's probably a good practice to stick a timer on scripts so you can get a feel for what is and isn't important. There's a handy function in the php manual (look under getmicrotime).