In most applications where pendants are placed above islands, it is more common to place the pendants in a symmetrical configuration centered directly on both the long and short sides. However, this is ultimately a personal decision that is dependent on your priorities and on the visual relationship of the pendants to the space.
If the island is going to be used for reading or working, then placing the pendants closer to the bar stools will be useful. However, if persons seated at the island will be there mostly for eating and conversation, it will be less important to move the pendants closer to the seating.