It's actually a random chance to get either ability, so it's like a 50% chance to get either one.
And as for which ability is better for Porygon, it really depends on what type of Porygon you make.
- Trace is really kind of helpful, but can be useless sometimes.
- Download is useful only when you make a Porygon mixed attacker (probably you'll make a Special one).
If you have a Mixed Attacker Porygon, you can take advantage of the foe's defensive weaknesses, since it raises the enemy's lower defense stat one stage.
If you are just a Special attacker, Download will be useless against enemies that have lower defense, if you're special, and will be of no help for you.
Either way, I believe Trace is the better choice, just because you have a chance of copying a really helpful ability, like Levitate or something.
Make a smart decision! ~