Unless the spiral cut gears are hypoid(the pinion is mounted off center to the ring gear), the straight cut gear will be a bit stronger, they also have less rolling friction.

Spirals are quieter, though, and I've heard that they handle high speed better(not sure if this is true or not).

One thing to think about is that while passenger cars and light trucks use spiral cut gears in their transmissions, medium and heavy trucks generally use straight cut gears.