The world of warcraft RPG is essentially just a campaign setting for Dungeons & Dragons. It has Warcraft-specific classes, magic, equipment and the like, and the rules are otherwise identical.
Now as to which is 'better'? Vague. The Warcraft setting, with a creative Gamemaster, would make an interesting D&D game, with very different flavor from the norm, but it's not nearly as supported with extra information, background and rules as the core D&D game.
If your money is limited and you can only pick one, go for the D&D Basic Set.