Identical. They're basically the same chip, but apparently the Xeon is actually higher binned and might even overclock better.
The X3350 has the same clocks as a Q9450, not the Q9550. I think you were mistaken. The Q9550 has a clock of 2.83GHz Vs. the clocks of the X3350 and Q9450 of 2.66GHz. They are basically the same chip, but at different clock speeds and different multiplier.