Have you thought about when your child will start kindergarten? In Massachusetts the majority (like 95%) of school districts put the cut off at Aug 31st, but in our school district is happens to be Dec 31st.
The private school DS would attend has an Aug 31st cut-off and DH wants to start him with the later year anyway. I would prefer to wait until he's closer to kindergarten to make that sort of determination.
What's the birthdate cutoff where you live and do you plan to abide by it?
Re: S/O Schools
We are waiting until she is almost 6 for kindergarten. The cut off is September 1st, you can test them and start early if their birthday is before Dec. 31st. I even held Audrey back and she is August 11th (it was a very good decision.) 
DD will be nearly six when she starts kindergarten. I'm okay with this.
DH and I were both physically late bloomers and it looks like DD is going to be on the smaller side, too. I think being one of the older kids in her class will make her feel
Burned by the Bear
Here they must be 5 on or before October 1st. No exceptions. So DS will be starting kindergarten shortly before turning 6. That works for me. I'd be wary to have him start any younger than that.