I have a 'fat' PS3 and the noise coming from it is very quiet. I can only hear it during the loading time between games or when no music is playing, even then it's still quiet.
It completely depends on which one you can find cheaper, my guess would be the 'fat' and as it's bigger it should have better airflow. If you want backwards compatibility with PS2 games, make sure you get one which supports it as not all do. I think it's all PS3s with a 60GB+ hard drive, it was the case when I bought mine anyway.
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