Numerically a high Discipline is probably better. The fact that it's your targeting ability is absolutely key. Also, since those 1 stress spells roll up a wizard will start taking consequences after casting his fifth spell one way or another. It doesn't matter if you cast a five 1 stress spells, or a 5 stress, 4 stress, 3 stress, 2 stress, and 1 stress spell.
That being said, since the relation between Discipline and Conviction says something about your character's personality I'd just go with what feels right, and let the numbers work themselves out.