Your choice will more or less be decided for you by your other unit selections. If you are running out of points just take Dark Elf Warriors to act as a missile screen.
If you are facing an enemy strong in missile fire units such as Wood Elves, High Elves or The Empire then you might like to take Corsairs which benefit from the Sea Dragon cloak and light armour (4+ save against missile fire). They are quite cheap at 10 pts and get 2 attacks thanks to their additional hand weapon.
Dark Riders are a must if you have no Cold One Riders or chariots. Use them to weed out Skirmishers and to outflank and take on enemy war machines. You will need them to march block your enemy and encourage them to break formation by charging you. Do not engage rank and file enemies with Dark Riders, flee, rally and reform for an immediate move instead.
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