Wow--these are exactly my thoughts for my 4th grade dd (except that I don't have specific schools in mind b/c don't know where we will be). I am 99.9% sold on single sex for a girls school. The math would be a concern...it just doesn't make sense they wouldn't allow her to accelerate in math. But it may be balanced out by the benefits of being in a single sex environment. For me personally, the boys in my middle school, (where there was ability grouping, so these were the gifted/bright high achievers) were a huge detriment academically and socially. I don't think that in an all girls environment there would be the same pressure not to appear to smart, not to answer questions, etc. etc. (Not to mention the distractions of all those hormones floating around) So I would lean toward the private, perhaps afterschooling or getting a mentor/tutor for math.
The reality is though that we will not be able to afford private school in most areas of the country, and I pray that we will have the kind of public school option you have. Good luck with the decision!
p.s. I already have my eye on the PEG program at Mary Baldwin--I think it could be a good fit for my dd.