Release notes - Getting started - AppImage should run on all recent linux distributions - see FAQ. - for Ubuntu users its recommended to install native package from NixNote PPA - although the AppImage version will also work. You can check the CHANGELOG, to see list of included changes.
NixNote2-qt511-x86_64.AppImage is experimental build with Qt 5.11 and QtWebKit, so far this is only recommended in cases where the standard build (Qt 5.5) doesn't work because of specific Qt bugs.