Either way is fine. Don't overthink it too much... Would you rather follow the mob to SDMT? Or veer off to an empty part of the park? You really can't go wrong, you'll get on lots of stuff. You say you're only going for about 5 hours, but that's our typical stay in a park. We get on tons. By being there pre-open and then the first 4 hours of regular open, you'll have the lightest crowds possible. And provided you have FastPasses booked for the last 3 hours of your morning, you won't be waiting for pretty much anything.
How I would plan that, depends on whether you have a FP to SDMT and PP. Ideally... FPs to SDMT and PP during your last 2 hours of your day, so you can hit up Tomorrowland in the morning. Then transition to Fantasyland, use your FPs, wait out Dumbo in between the two, and you're good.