I would always go with the better quality pump. Engine cooling is just too important to gamble with, and the price for labor involved offsets any $$ difference. That said, it doesn't mean that aftermarket pumps are necessarily of lesser quality. (I have used aftermarket pumps with no issues.) Just don't get one made in China. (There have been many reports of failure with Chinese pumps.) If you can't establish where the cheaper pump is made, and someone to vouch for its quality, I would go with the OEM product.
Best regards,
Best regards,