I recently used Agave and didn't notice any particular flavor. Just a source of sugar with some color. The recipe you link gives the option of honey or corn sugar. Corn sugar doesn't have much character. Maybe any type of sugar would fill the bill. For caramel flavor I like to use caramelized sugar. Just heat it carefully to boiling in a heavy duty pan, and get it into the boil quick! Because it continues to cook and rise after you remove it from heat. Be careful.