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I have not played with Nrg2. I have a year and a half on X-One Biphase, so I cannot give you any play test comparison.
What I can tell you is that the USRSA machine testing results have these two very, very close to each other. According to this one source, the X-1 biphase is 2% stiffer and loses tension 5% faster, than Nrg2. This kind of slight difference is well within a normal range of testing error. The big picture is that the material of the string matters much more than the design of the string.
As to the previous threads mention that a stiffer string creates more control, that is open to debate. Some would argue that since a softer string creates a longer dwell time, or deeper pocket, that this creates control. I do know from experience, AND machine testing data, that X1 Biphase is among the softest playing multifilament nylons available, and will hold tension very well right up until breakage.
Best regards
-Jack