As others have said, the 757 is likely your fastest progression to a line on paper. That said, the company hasn't been putting many(any?) newhires into the airplane for awhile, preferring to send them to the 767 instead. If they don't hire much into the 75 in the near term, it's not gonna help your relative seniority. The 76 has more senior people transitioning out of the MD and Airbus every bid, in addition to the recent hires move from the 75 to the 76. In the end, the Airbus may turn out to be the quickest to a line for you as they're hiring into it and there aren't many senior FO's transitioning into it.