Acctually its not competing if they're both from the same company...thats like them saying "Oh my god, don't buy WoW. It sucks. Go buy D3." Then they turn around and say "No, D3 sucks. If you want a good game buy WoW. D3 sucks don't buy it even though we're the creators".
See its that same as them releasing Brood War and Diablo 2. No one said they sucked. In fact mostly everyone bought both of the games...