The FX-55 will be slightly faster. Since the fx-55 uses normal unbuffered ram and it is very easy to find unregistered low latency ram.(eg cas2)
With the opteron you must use registered ram which is slightly slower and harder to find in low latency flavours.
Side by side with equivalent memory timings the the fx-55 will still be marginaly faster due to extra register(?) operation required when opterons access registered ram (though it is only a very small difference)
There is also one other important difference, the fx-55(939) can only be expanded to 4Gb but the (1x)opteron can be expaneded to at least 16gb.