What's the best way to control costs in health care? Is it more competition between a larger number of hospitals and doctors in a geographic area? Or is it better coordination within bigger but fewer provider networks? With pressure mounting to rein in medical costs, that debate — seemingly more suitable for a health policy seminar — forms the backdrop for the mating dance underway in the heath care market as hospitals across the state and nation seek to consolidate. And it colors the deliberation of regulators weighing a bid by the region's largest hospital and physicians organization, Partners HealthCare System, to grow even bigger.