Power switch: leaving battery connected at one terminal
If your single switch truly completely disconnects one battery terminal, then there is no difference in self-discharge of the battery.

However, this is one of these cases where there might be a difference between ideal circuit theory, and messy reality. In messy reality, there might be other conductors appear across your single switch that you hadn't intended. Condensation perhaps, a rat or spider making a nest, or a dropped spanner from a careless mechanic. Then, having both terminals disconnected might improve things.