I think the fact that Tiff works for Channel 5 would almost guarantee that he isn't the Stig.
Who's quicker? Tough call. Tiff has been a grand prix driver, but I think most of his experience is with endurance races. So while he's definitely quick, he's perhaps more accustomed to conserving a car and driving it consistently. The Stig definitely drives more like a rally driver, so far as I can tell. He's very much in control, but he seems pretty aggressive with the wheel and throttle.
I'd have to say the Stig is quicker, but I'm not 100% convinced.