MLK weekend is MUCH less crowded than President's Day weekend. Many school districts are closed President's Day week, but schools are usually only closed for MLK day, not the whole week.
Weather wise, there's no telling either time. In both cases you could have days in the 80s with cool nighjts, or you could have mornings near freezing with afternoons in the 40s. Winter weather in Orlando is completely unpredictable!
Bob