I don't think any 20" wheel folder is made for off roading. The problem is while hitting an obstacle ( or a hole ) a small wheel bike can throw the rider off. Therefore the rider has to learn to move their weight behind the saddle when necessary.
Not trying to toot my own horn, but it will be hard to get a better off road 20" bike than the 9FS. However I do like the Airnimal, not so sure about the BF in the trails.
Thanks,
Yan