I'd say the Jets over Cowboys defense without question. Not only because of the coaches, but because of the overall talent. But yeah, I'm not sure if Rob Ryan has ever coached a defense that's ranked ahead of his brother Rex Ryan's defense. Just double checked, and no, he hasn't.
Rex had had defensive rankings of 1st, 1st, 2nd, 3rd, 5th, 5th and 6th.
Rob had had defensive rankings of 3rd, 14th, 22nd, 22nd, 27th, 27th, 30th and 31st.
But in reality, does any of this really matter heading into the 2012 season? Dallas doesn't play the Jets this year. The Cowboys aren't playing a single AFC East opponent this season. Although they do face off against 4 AFC teams in the Ravens, Steelers, Browns and Bengals... They're pretty much irrelevant to the Jets here in 2012.