In addition to the test, I would suggest a scheduled meeting with your kid's preschool teacher. Really talk to them and see what they think, not just a conversation in passing as you're picking them up. They'll have a lot of insight, particularly on the maturity issue. My daughter is July 31, with the cutoff Sept 1, so will either go to Kindergarten next year as one of the youngest or we'll hold her back. So we'll be in the same boat. I have no doubt she can handle the academics, but a little more concerned about the maturity issue.
For us, school choice may factor into the equation as well. Our zoned public school is unacceptably terrible. So we'll try to get into one of the local charter schools, and if we don't that would weigh heavily in favor of holding her back.