Generally, you debate (discuss) a topic, but argue (put forward) a single fact/statement.
1) "Whether or not I am the best singer is debatable" (Topic = 'whether or not I am the best singer')
2) "It is arguable that I am the best singer." (Proposed fact = 'I am the best singer')
People do use 'arguable' in 1), but I think 'debatable' is nicer.
you can't use 'debatable' in 2).