Of the two I would pick the MLK weekend. You will have some more time to plan and while it will be crowded, it probably won't be nearly as bad as Thanksgiving/Christmas/July 4th. Since it was just the holiday break parents are less likely to be taking kids out of school for a vacation at that time or taking time off of work. But I would still have a touring plan and be prepared to be flexible and have some waits.
Weather will be cooler, you may not want to swim but it will be nice for being in the parks.
What are your priorities for your visit? Ride the rides? Enjoy the shows? Eat in the restaurants? Relax at the hotel? Do everything? This can make a big difference. For us we don't eat at the sit down restaurants so being able to make advanced reservations is no big deal to us and wouldn't factor into when we would go. But if my goal was to try to do every park and be able to go on every ride and see all the shows then a holiday time would not be possible. If you don't want to do it all then I think you should go for it!