Your nephew will perform to his potential whichever of the two schools he attends.
However in my experience, Priory is normally the last resort for a DC when there is no other option available (i.e. they have failed to get into any GS).
I would go for HGS without hesitation; having Grammar School on ones CV will be a substantial bonus going forward in life. I know many employers (lawyers, accountants etc) that dismiss CVs outright if the applicant hasn't attended a grammar school.
I have to say that having Grammar School on ones CV adds nothing to your ability to gain employment with some of the biggest companies. I do know that many require potential candidates to have obtained a degree from a Russell Group university but most, if not all, will look at the classification of your qualification rather than where you obtained it.
I have to say I'm with um all the way on this. Such a shame people have these unfounded opinions.
I have a son already at Handsworth Grammar and another who will hopefully be starting at Camp Hill in September but personally I never attended a grammar school nor a Russell group university but that hasn't stopped from gaining employment with some big companies. I have never been out of work and been able to go from one job to another without much difficulty but at my level its more to do with working experience and skills rather than what grammar school I didn't attend.