Clement Lefebvre, the leader of the Linux Mint project has announced the availability of the beta build of Linux Mint 18 Xfce, codenamed Sarah. The Xfce version is very similar to the Cinnamon and MATE versions which shipped at the end of June.
While the release comes with the new X-Apps, the Mint-Y theme, new artwork, an Ubuntu 16.04 base, and version 4.4 of the Linux kernel, it still runs Xfce 4.12 and MDM 2.0, both of which were present in Mint 17.3. The reason Xfce and MDM are at the same versions is because they are the latest upstream versions. They'll likely be updated with new point releases in the Mint 18 cycle.
Interesting to note, the blog post mentions that it will be possible to upgrade from Linux Mint 17.3 to Mint 18 and that instruction on how to do this will be published after the final release. It's interesting because only last week the upgrade paths for Mint MATE and Cinnamon were published but it wasn't clear if other spins would also be able to do an in-place upgrade, but now we know they'll be able to.
The last spin we are expecting to hear about is the KDE version. It is the only one which hasn't been mentioned by the Mint team yet. We can probably expect to see Mint 18 swap over to the newer KDE 5 environment.
Source: Linux Mint Blog