I agree that age 6 is helpful to wait for... having the options for extended norms if you have a "way out there" kid can make a difference in scoring... also being able to do that GAI and leave out a processing score that makes child looks less able may matter if that is a significant difference for him.
My child is extreme on both ends and WPPSI at 4 had him hitting lots of hard and soft ceilings which made it less informative. Processing Speed was significantly lower and did impact his scores negatively. There was no way to filter out that effect on WPPSI.
At 6, taking the WISC gave him more room and we got more information. His processing gap had increased and on WISC it makes his full scale IQ just under 130 (cut off for many gifted programs). Having the option of a GAI calc on the WISC however shows that he is in fact profoundly gifted.
It's hard to know which kind of child you have before testing (that's really the point isn't it
) but what I've seen is that for most the broader and more adaptable nature of the WISC provided more helpful information.
Good luck with your testing!
