G3's reply made me forget that there are other options sometimes. Are there any GT schools in your area? My ideal (for my kid) would be to with other kids his age, but who are also at his intellectual age. Don't have this option where we live now, but G3 is so right. Any decision you make will be based on what you think is best at the time, and you will possibly (probably?) have to make changes in the future.

And I forgot to answer the question about testing. To help us in our schooling decisions, we had our DS privately ability tested at age 4. The local public then did their own achievment testing last spring.