Hi mon2two,
My name's Chris and I'm a school psychologist. I wish I did more testing for giftedness, but unfortunately I don't get that chance often. I do give both the WPPSI and the WISC on a fairly regular basis. The benefit to your child taking the WPPSI is that, because the test has such a small age range and is geared toward younger students, there are more questions that test what they are supposed to know. For the WISC, commonly a student at the beginning of the age range allowed by the test will receive a high score based on his ability to answer 2 or 3 questions on a subtest rather than 6 or 7 focusing on more age appropriate topics on the WPPSI. This causes scores to fluctuate fairly wildly based on one or two incorrect or correct answers and is somewhat unstable in my opinion.
It is also fairly common for a student who is only 5 years old to have an unstable IQ score becuase of a lack of maturity and understanding that he or she must do their best throughout the course of testing. Kids that age don't see it as a problem to take one or two questions off and just give answers of "I don't know" rather than rask their brain and try to come up with something if it doesn't immediately come to them. This may be a reason for the drastic differences in IQ scores that Jane mentioned for her kids.
One of the issues with testing and quickly re-testing is that the student deveops a familiarity with the items. Even if the examiner decides to now administer the WPPSI, it's still a Wechsler test and the subtests are quite familiar in design, even if the questions are different. On tests like the block design, having some familiarity with the blcoks and how they can come together to form a certain design, for instance, may result in higher scores than when taken previously. In regards to the Stanford-Binet, if the psych is uncomfortable with that, maybe she is familiar with the Woodcock-Johnson 3: Test of Cognitive Abilities.
