Unfortunately, when Xur does sell it, it will probably have better attributes than the rng version. I Xurned the titan arms, and then got them in the daily just after with my Hunter. There was about a 10 point difference in the ability between them, the Xur purchased being better. So, even if he sells it, look and see if it's worth buying to replace the one you have.
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Xur stats are pretty random as well. The first two times he sold sunbreakers they had high int, the last two times it was low. The double nade chest piece for titans has always been low compared to my RNG one.
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That is the sad truth
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I bought the VGV from Xur and my friend told me he got same from either an engram of a mission reward and it had better stats then what Xur sold. So I think the stats are rng based as well