It must be different in Dallas. I work in Dallas ISD, and you can pre-register, but you can also just show up the day of, or the week before, or even after school starts. There is not an issue of the school being "full." No kid ever gets turned away. If a grade level is over crowded, the school adds an extra teacher or moves one from another grade level.
There is no penalty or problem at all with not pre-registering.
With that being said, I say definitely pre-register at the school you are currently at. If there is an open house or a chance to talk to the teachers or principal, go on up and introduce yourself. Describe your child's personality and ask that they place your child with the best teacher for him.
If you have a chance to go to an open house at the other school, I would do that as well.