If you've already installed Reaction, then the Standalone is the correct choice.
Also note that you may have to update your Gantry versions as well if you switch template to a later version.
BUNDLE
This is the bundled template package for XXX that you install into Joomla - Admin → Extensions → Install/Uninstall. It contains both the template and the Gantry library.
Do NOT use this package if the Gantry library has already been installed, either through a bundle package, or through a standalone Gantry install.
STANDALONE
This is the standalone template package for XXX that you install into Joomla - Admin → Extensions → Install/Uninstall. It contains just the template.
Use this package if Gantry has already been installed, whether by a previous bundle install or separate independent Gantry install. Gantry is required for the template to work.