We had our son tested just before he turned 5, the spring before he should have started kindergarten. He was first assessed with the Woodcock Johnson Achievement tests. With those results (all 98% and higher), we headed to the Belin-Blank Center in Iowa for testing to provide the school with information, as we were requesting early entrance to first grade. There, he was tested with the Standford-Binet 5 and then an out of level test for end of the year first graders (Comprehensive Test Program by Educational Records Bureau). The examiner also tested him for ADHD and general adjustment issues at our request, as the school was concerned about "maturity" and ability to handle first grade. The combination of tests was enough to get him into first grade.
You might want to check out articles on Hoagies about why test scores vary. I don't know much about the two tests your son has had. There are many reasons why children's test scores might vary. And the older the child, the more reliable the results, so I've heard. It has been suggested that our son be retested at the age of 7 or 8 as his test scores are likely to even out more and be more "true" as far as IQ.
Hoagies page also has nice descriptions of all the different tests available.
Are you considering early entrance to first grade or looking for differentiation? Is the testing going to be done by the school district or privately? If privately, you should look for someone experienced with testing gifted kids, and interpreting results for gifted kids. It's important to look at the big picture, not just the final scores.
