Aura sphere never misses, has 20 pp, and power of 90. It's a a special attack.
Focus blast only has 5 pp, and 70 acuracy, but it has a 120 power and may decrease the oponents' special defense. It also is a special attack.
I'd go with aura sphere because it's a relable move as it never misses, as well as more practical pp wise, and also is decently strong. Of course, if your fighting style is more risky, I'd go with focus blast.