Yes and yes. Always dup, and it will always tell you you’re about to do a distro upgrade.
That message is important on Leap because there is a difference between upgrade (up) which installs patches to installed packages and distro-upgrade (dup) that would move from major/minor to the next major/minor (ie 15.5 to 15.6) which is a very different process in Leap.
Tumbleweed is released basically daily as snapshots. Each snapshot is akin to a major release in a traditional distro, so you have to use distro-upgrade to do the update properly. That is also why up doesn’t do anything, a snapshot isn’t updated after it has been created so there are no packages to upgrade in that release.