Complex sentence: As has been customary since 1993, the Royal Rumble match winner received a match at that year's WrestleMania, (in this instance: Wrestlemania XX) for his choice at either the WWE Championship or the World Heavyweight Championship.
Split into simpler sentences: As has been customary since 1993, the Royal Rumble match winner received a match at that year's WrestleMania, in this instance Wrestlemania XX. Beginning with this Royal Rumble, the winner could challenge at WrestleMania for his choice of either the WWE Championship or the World Heavyweight Championship.