Other places that have that progressive bent:
Capital City Public Charter School (it's free, but hard to get in to).
Georgetown Day School
Burgundy Farm School
Sandy Spring Friends (maybe a little more traditional than the others on this list)
If you like Montessori, you might look at Christian Family, and Barrie, both of which have a crunchier feel than say Aidan or Franklin.
I've heard mixed things about both schools you mentioned. I feel like Oneness can't decide what it wants to be -- blending Montessori (very academically focused, very practical, emphasis on independence) with Waldorf makes me scratch my head -- it's like deciding that arugula and creme fraiche are both trendy so you're going to stir them together.
Waldorf, I'd investigate closely who the teacher is for your year. I know people who have been happy there, and people who felt their child didn't click with the teacher and then had that teacher for years. If you're committed to private education for your children, in the DC are where admissions are super competitive, moving out of Waldorf in the early years can be very difficult because the kids are academically behind at that point -- yes, I know they catch up down the road, but I think private schools are unwilling to take that chance.