How do you measure the scientific capability of a radio telescope?

Advances in observational astronomy inevitably require the design and construction of larger and larger telescopes in all wavelength regimes. Larger receiving apertures imply greater sensitivity, but is sensitivity the most appropriate metric for all science objectives? The Square Kilometre Array (SKA) community is currently grappling with this issue, and there are suggestions from some quarters that the SKA receptor area might not need to be one million square metres in extent, and that aperture can be traded for field-of-view and bandwidth. The choice of metric for optimizing the performance of a radio telescope will have an impact on the observing programme for the instrument, which in turn will affect the sociology of the scientific community associated with it. The various metrics that are applied to rank radio telescopes will be discussed, and their relevance to the science objectives of the SKA and MeerKAT radio telescopes will be investigated.