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In the main game, you get a sword past the point of no return, which can be upgraded in HoW, for use against the end boss. This would suggest that you should finish IWD first, then do HoW next, if you use long swords (I do). The problem here is that your import save is before the final fight, so your xp/loot (if any) from that fight doesn't carry over.
However, you get items in HoW that can be crafted into useful gear in main-IWD, which would suggest you should finish HoW before finishing the main game.
I guess I finish the main game first for the sweet long-sword.