In the best interest of the child, since you already found the child frustrated by unchallenging math classes, pick the more academically focused school with the caveat that, you should go and see for yourself if it is more challenging. People's ideas and the ways they communicate about what constitutes a challenge differ, so go to that school and see for yourself. Also, listen really hard to discern if the more academic school understands the different way gifted people are socially and emotionally.
With the more challenging, more academically focused curriculum you can save yourself frustration every school night. Because what happens, as a general rule, is that your gifted child will be on their own, not learning anything new in their school day while the teacher teaches the other children and the thing that will irk you is that the teacher will not appear to care. So, we give them the benefit of the doubt that they think their job is to get everyone in their class to pass that grade level. They are not trying to give a full year's worth of learning (at the gifted child's level) to the gifted kids. School is not currently set up to do that.
What happens sometimes when the gifted child skips grades is that then their maturity level is below their mental capability (asynchronous development) and they might not be mature enough to make 'good career decisions' when they turn 16 years for example and are making decisions about college. (And, that is a loaded question depending on whether or not you think the human brain 'needs' a college education.)
The way they teach math is so slow that it is painful to us. Math in my mind requires immersion, history, walking in prior mathematicians footsteps virtually so that you are trying to see what they saw and follow their discoveries and see if that helps lead us to 'new' discoveries that might be helpful or just really interesting to know.
