If your child was admitted to both, congratulations. Nice decision to get to make.
My child is at WIS. We looked at Maret but did not apply for a variety of reasons. In the end we decided between GDS and WIS and are very happy with our decision.
From talking to other parents over the years, I have the impression that none of the NW private schools are incredibly strong in math and science, particularly in the lower grades. Each manages to have a handful of very precocious students in those areas anyway. If those are your priorities, I would spend some time studying the curricula of both middle and upper schools and speaking to those principals. Good luck deciding.