The public option is gone. Expansion of Medicare is dead. But an intense fight continues over a crucial issue in the proposed healthcare overhaul: how far Congress should go in emulating the type of insurance marketplace that is at the center of the pioneering Massachusetts insurance program. Called "exchanges" in the federal health bills and modeled on the Massachusetts Health Connector, they would enable people to compare and purchase insurance as easily as they shop for airline tickets at an all-in-one travel Web site. The concept of such comparison shopping is generally backed by insurance firms, but exchanges are also controversial because Democrats in Congress want to use them to impose greater oversight and cost controls on insurance companies, requiring them to provide certain levels of coverage at lower profit margins.