I've shipped priority all of one time, and the only reason I decided to was because of warm weather, and I only live an hour or so away from my transhipper. For the most part I drive to pick up the fish from her myself, so it saves me around $40.
I always recommend express because of all the factors playing on the fish that can be potentially lethal. For example, if the breeder didn't fast the fish, if there's a leak in the bag, if the weather is too hot or cold, if the box gets stuck next to the air conditioner, and not to mention the amount of stress already on the fish from being boxed up, on a plane, and being in complete darkness for several days without food.
Not to mention that most breeders DO NOT offer a live arrival guarantee for priority- that means if your fish dies, you're out the cost of the fish AND shipping anyways.
Always use express.