Lightning Arrow Build - Which Gem combination? [Fork VS Chain]

You don't have to stick to one combo or the other. Fork works really well on mobs that cluster together while chain works well on less clustered mobs. Switch them around depending on the map/area you are farming. Fellshrine, or example, is a great location to be using fork.

Others have explained why chain is superior to fork in every way. But a good way to look at it is fork is a poor mans chain. Fork is still verry much viable but its also about half as powerfull.