This happened to me a few years ago when I was the captain on an MD-11. We were sitting number 1 for takeoff, holding short perpendicular to the runway, onto which we'd make a left turn to align (the first officer had an unobstructed view of the final approach.) Anyway, about 3 miles out was a 737, on approach, when the f/o says to me "look at that, his gear is not down." The f/o then, for the next 5 or 10 seconds, kept talking to me about how dumb those guys were to be so close in to the runway without their gear. He (the f/o) started getting animated, when I picked up the mic and broadcast on tower freq "GEAR", at which point the guys landing gear came down. We said nothing else, and they landed and taxied off while we took the runway for our takeoff.
I guess my point for telling this story is that while seniority may not be a perfect indicator of experience, it is overall, a good indicator. That's not to say that there aren't first officers who are more experienced and more mature than the captains they're flying with at the time, but overall (imho), captains are more mature and more experienced than first officers, especially if you look at their experience at a particular airline. Granted, guys on their 2nd and 3rd airline, bring a ton of experience to their new carrier, but for recent experience, at that airline, the captain usually has more.
I believe that there's a majority of airline pilots out there who would have performed as well as Captain Sullenberger did, both captains as well as first officers. And I'm glad of three things: 1) that everyone survived, with no real physical injuries; 2) that Captain Sully represented our profession well; and finally 3) that it wasn't me flying that jet, that day.
JJ