It's certainly impossible to know and only having both would really answer the question of which is better. However, with an 8 yo, I'd do the WISC. All the really big scores I've seen on the SB5 have been with pretty young kids (5ish).

I think the WISC scores easier for verbal tests which might be a bonus. I'm not at all sure how math or math thinking gets represented on these tests (and maybe it doesn't as a separate thing). There are so many ways to have math ability as an 8 yo from great reasoning to great calculating to great ability to figure out the logic verbally. Some great math ability might show up as verbal precision and logic whereas other kids might do better at nonverbal tasks.

I have very mixed feelings about SBV.

A little bit of background. Despite knowing better we had DS5 take WPPSI in summer. Both his PIQ and FSIQ were in the 99.9% but lower than the DYS cut off (150 for WPPSI). His VIQ wasn't too far behind.

DS5 took SBV a few months ago. On one hand he got qualifying DYS score in the nonverbal section. He also scored more 19s on SBV than on WPPSI. On the other hand he had low scores on three consecutive subtests. He ended up with VIQ significantly lower than his WPPSI VIQ which brought down his FSIQ.

I don't think those 3 subtests clearly represent his abilities as they differ greatly from his WPPSI results and his other two SBV verbal subtests, which were both 19. Was it the test? The tester? DS5? The age? Most likely the latter two. My guess is that he had had enough by then since all the subtests were towards the end (#7-9 out of 10).

If you are looking for DYS scores I think WISC is a better bet with the extended scoring as well as the GIA option available.

I agree.

Thank you, everyone.

I think I'll stick with the WISC as well. I appreciate everyone's input!