For years, the federal government has spent billions to entice senior citizens into private Medicare Advantage plans run by private insurers that offer bonus care for low premiums. Now that nearly a quarter of Medicare patients have enrolled in these health plans, including 350,000 in South Florida, Congress is poised to pass legislation that would yank billions of dollars from them. Many patients resent this turnabout and want to preserve the program, which offers extras, such as vision care and gym memberships. On the other hand, patients who pay for care in other ways say they resent subsidizing those who get gold-plated treatment at bargain rates. The controversy over Medicare Advantage is one of the thorniest issues complicating final passage of landmark legislation to overhaul the nation's healthcare system. Republicans have turned potential cutbacks into a rallying cry against reform.