I would defiantly go with the 5400PRM, unless you follow aibo82's advice. I ordered the 7200RPM myself, but I think it's too noisy and not worth that little extra speed.
So now I have to find another harddrive and either use my 7200RPM as external harddrive or sell it. I wished I took the 5400RPM now.
aibo82, will the noise be the same with Momentus or will it be lower than the Apple provide? They are both 7200RPM, so I am a little worried it will.