The instability is a big issue with the young kids just by nature of their age and uneven development. Also, you have more chances of the child goofing up, getting tired, wanting out, etc.
Another difference is that the WPPSI relies more on crystallized intelligence (that is knowledge already acquired) than the WISC. So a child who just by nurture has been exposed to more vocabulary and information will outperform similarly smart kids that have not. These smart kids will probably "catch up" in those areas in due time.
Neither test is too long, at least I don't think so. The number of sub-tests used in the wPPSI increases with age. WISC doesn't change in that sense.
We absolutely tried a different tester as I said, the first one did not handle things well from our perspective (was somewhat confrontational to my child while I was still in the room and they were building rapport).
The report from the first tester was somewhat generic or rather formulaic. Child is _ years and __ months old and lives with ____. She has attended preschool since ___. In many phrases it seemed like she was just filling the blanks. The second tester presented a more descriptive portrait of my child. She included many observations of how she handled the test, what she could see in personality, approaches to problems, etc.
