Control (Discipline) bonuses are more useful than power (Conviction bonuses) when using evocation.
First of all, the normal combat mechanics favor accuracy. To use an extreme example, let's compare a Weapon 2 with 8 to attack versus a weapon 5 attack with 5 accuracy (a more extreme 8/2 split would involve taking backlash). Each extra success on attacking adds to damage. When the weapon heavy attack barely hits against a defense 5 target, it does 5 damage. Against that target, the 8 accuracy attack has 3 successes, so it hits for weapon 2 plus 3, also 5. If the target's defense is 0, then both attacks hit for 10 damage. And if the defense is more than 5, then the accurate attack is the only one doing damage. So accuracy, and thus Control/Discipline, have a clear advantage because they'll do at least as much damage as a more power shifted attack.
Next, spellcasters have an easy way to get additional power beyond their Conviction. Each evocation with some power to it costs at least 1 stress. And you can overchannel to get additional shifts beyond your Conviction by taking additional mental stress. After you have 1 point of mental stress, both a 1 point hit and a 2 point hit will fill in the #2 mental stress box. So you can essentially overload your spells for free: a caster with 4 mental stress boxes can cast 4 evocations at power; or 1 at power, 1 at power +1, one at power +2, and one at power +3. Gee, which one sounds better?
Of course, you need to have enough control rating to handle that boosted power without taking backlash or wasting it in fallout. Having control somewhat over power helps with using the extra juice safely.
Of course, someone planning on attacking with magic a lot probably wants Conviction 3 as a minimum to get 4 mental stress boxes. And both skills are obviously important!
However, with a Superb skill cap, Conviction is worth another look because it can grant another mental consequence. OTOH, you might need it, since it would be harder to control your spells (without a control heavy focus item).
Note that I'm only really looking at how the skills affect spellcasting. Maybe you like conviction for some other use, as well as casting. However, it seemed like Discipline could be used as a defense against many mental and social attacks, whereas Conviction comes up mostly for the stress track and the Holy powers, as far as I remember.
Unless I'm missing something.