Both schools will have lots of outdoor time, no tech/low tech, and an emphasis on whole child development. But they are quite different in other aspects.
Butler is a Montessori school so it has mixed age classrooms (a three year age range) and class sizes in the mid-20s. At Butler kids are in the same classroom, with the same teacher, for three years. The teaching approach, structure of the day, and classroom materials must all align with Montessori standards as part of its accreditation. Children do small group lessons and are supposed to do follow up work, or practice, based on the lessons. Some kids really thrive in this kind of environment and learn a ton, but others spend a lot of time wandering aimlessly around the classroom and chit-chatting during the work period and don't end up learning much. Butler does not give tests, grades, or homework. Kids have to take standardized tests, but the school will not share your child's scores with you. It is very hard to know what your child is learning at Butler.
Seneca is one grade per class, and each class is about half the size of Butler's. Teachers give lessons to the whole class, with some breakouts for reading and math. It is an IB school, which gives it an approach to learning but does not dictate the curriculum. For your preschooler, Seneca classrooms are going to have some "toys" in them, whereas Butler will only have Montessori materials. Seneca is also inquiry-based, which really develops the kids' ability to ask questions. Seneca uses specific curricula for math and reading based on what they think are the strongest curricula available.
I would ask both schools about if/how they could accommodate your 2e child. I would consider your children's interests and temperaments (is your kid intrinsically driven to learn? -->Montessori; does your kid love imaginative play? need some structure? --> Seneca, etc.) and your family's approach to education in general and decide based on those. Good luck!