The apparent inability of some 'pilots' posting here to determine which instrument is correct, when there's a split as large as 15 degrees is a bit alarming. I'll be charitable and assume you're all non instrument rated and never fly airways.
Generally speaking this avionics engineer would put more faith in the HSI than in the Standby Compass. While STC is correct to say that you might have a +ve error on one and a -ve error on the other, there should never be enough to give you a 15 degree difference unless you're reading the standby with something that influences it switched on. Redsnail points the way...
To work out what you should switch off before reading the standby compass, try reading the AFM, there's often a clue or two in there; not just about compasses either...
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Through difficulties to the cinema