Which will it be: an orbiter with a precise high-resolution camera or a large rover that lands on airbags? NASA had planned to announce on Monday which of the two proposed robotic missions it will send to Mars three years from now. But the space agency decided Friday to delay an official decision. Edward Weiler, the NASA associate administrator, said the selection process was more complex and difficult than anticipated and that it will take perhaps one to two weeks longer than planned to make the final decision.