This question came out of reading the Blackest Night tie-ins for R.E.B.E.L.S., where Vril Dox found himself inducted into the Sinestro Corps, only to ultimately be dismissed because he wouldn't follow orders. Head-to-head, who is more dangerous, Sinestro or Vril Dox?
Sinestro has the power ring, obviously, though which version varies, but Vril has the intellect. Sinestro is no idiot, though, and Vril's more than capable of coming up with some dangerous weapon, given time to think. They're both arrogant enough to regard basically everyone else as expendable, though I'm not sure it's for the same reasons. With Dox, he tends to view things from a logical standpoint. If you have to sacrifice one person - or a whole planet - to stop someone who's conquered 9 galaxies, so be it. He's confident he's devised the best plan. With Sinestro, I think it's simple arrogance. He thinks he's better than everyone else, so they're less important than he is. Not that Dox isn't arrogant, but he's not going to throw away lives unless he has to. A person who is dead now is one who can't help him later, so better to avoid suicide strategies if possible.
At the same time, Dox is a little more ruthless. He'll do just about anything to win, including pleading with his son for help, making deals with interstellar despots, go drinking with Lobo, or hamming it up to the media*. Sinestro will do things he finds mildly distasteful - teaming up with Hal Jordan, for one - but I think he has too much pride. There are certain things he wouldn't do to win, because he'd consider them too humiliating. I don't think he'd play servile to encourage a foe to drop their guard, or to gain assistance. He'd be more likely to brashly demand submission, which isn't always going to work.
* After he captured Braniac (with a little help from Brainiac 5, slumming it in the 21st Century), he told the media he pulled it off because it's a law of nature, the good guys always win. Which is the sort of thing he'd normally sneer at, but it plays well with his potential clients.