In spike, or not refers to the stem (spike) that the flowers grow on. It's the inflouresence but is just commonly called a spike. So if your plant is going to bloom, it is in spike. Low spike is if it isn't too far along yet.
I find it less stressful to buy plants not in spike, then you don't need to worry so much about losing the spike or buds/flowers in shipping. I assume it's easier on the plants too. But i'm not sure.