Powell and McKnight are very close in size. I see McKnight listed at 5'11" 198 and Powell at 5'10" 205. Not enough of a difference to draw an inference anyway. I'd dispute the 'scatback' label too. At the end of 2010, McKnight started a game and ran the ball 32 times for 158 yards.
I have both in various leagues so I'm not expressing a preference due to ownership, and honestly have no idea which will be the better long term play. It opened my eyes when 4th rounder Powell was drafted a year following McKnight, also a 4th rounder. That sort of thing would seemingly indicate a fall from grace for the earlier guy, but McKnight maintained his role in 2011 and currently stands to be the one benefiting from Tomlinson's departure. Could Powell beat him out? Possible. Could Greene's eventual replacement be drafted in this draft? Also possible.
Bottom line is the odds are against either 4th rounder ascending to the starter role. It happens, but the odds are still against it. Owning either with the hope of a starting role in their futures is a bit of a flier. I'd make no assumptions with either Powell or McKnight and see how it plays out.
I think maybe they drafted him because they had lost Brad Smith and didn't really have a plan "b" because of the lockout. I remember an interview on Sirius shortly before the season started where Jets coaches talked about how that loss was BIG for them and they thought that McKnight was the only guy they had that they thought could replace those roles.So I thought that maybe they believed they needed depth there.
I don't know how mcKnight may have a better chance of getting away from that role and having a more fantasy-friendly role this year but I have seen a lot of Powell in college and I honestly think McKnight would produce better in that offense if the Jets had to turn it over to one or the other.
I don't know if the Jets will draft another RB this year but I can definitely see them bringing in a FA. A couple of guys that I could see would be Tashard Choice or Knowshon Moreno if he is let go. I don't think either of those does anything to clear up the fantasy questions but I could see ither of them being available and working for the Jets.