These aren't mutually exclusive. Even in The Terminator, Skynet's method of choice is nuclear war. Yudkowsky frequency expressses concern that a malevolent AI might synthesize a bioweapon. I personally worry that destroying the ozone layer might be an easy opening volley. Either way, I don't want a really smart computer spending its time figuring out plans to end the human species, because I think there are too many ways to be successful.