Our DD did the WPPSI at 4y9m and the SB5 at 5y4m. In her case she had ceiling issues on the WPPSI where she hit soft ceilings (she ran out of test before reaching her level, but made mistakes along the way or gave vague answers and so was getting 17s rather than 18-19s). This was in part due to not gel-ing with the tester and so preferring to give the briefest possible answer to questions where she could have scored either 1 or 2 and so often scoring only 1. Her FSIQ was 8 points higher on the SB (146 v 138).
Given that you don't have that particular problem the only benefit I could see for your DD would be that the SB5 has extended norming, like the the WISC, so if she is actually beyond 150 then there is a chance to show that. As I understand it the extended norms of the SB work slightly differently to the WISC. The SB is supposed to be a more mathy test, and good for the visual spatial types.
