From my perspective as a curious observer of both projects, OSTree certainly looks attractive, because it doesn't waste space on two rootfs partitions which have to be oversized to accommodate future growth of the image. I initially thought OSTree required btrfs, because Project Atomic used btrfs the last time I looked at it. But according to the docs, while OSTree will take advantage of btrfs features if btrfs is being used, OSTree itself will work with a variety of filesystems including ext4.

Edit: An upside of the alternating rootfs partition approach is that the rootfs can be cryptographically verified at the block level. Chromium OS implemented this, and CoreOS also uses that implementation. This is probably outside the scope of Mender itself, but the updating approach used by Mender enables it.