From William Candillon today on the "Yet another PHP blog", there's his comparison at the source coude level of PHPCompiler versus Yaxx.
The main goal of phpCompiler is to translate PHP code directly into Linux assembly code (and they are apparently very close to it).
He mentions, though, that phpCompiler just doesn't fit what he's trying to do. He needs things like portability between platforms and a simple way to extend the PHP lexer and grammer for phpAspect. Unfortunately, phpCompiler either doesn't allow these or just makes them too difficult. He makes his choice for his situation - Yaxx