I voted for the Chickenhawk. I can see scenarios where the Nighthawk would certainly be better but, in terms of where the game is today and what players (in my area, at least) are using...frankly...there's not enough Air Phase action to warrant the Nighthawk's big AA missile launcher (and it's associated point-cost) ...YET!
Also, if anybody has to worry about Air assets, it's NOT the MI. If the Arachnids want to attack you with an air unit (i.e., the Hopper) they have to come down to ground level to do it becuase it's only attack is close combat...meaning you don't need the AA to wack'em if they become a threat.
All that said, the mere presence of a Nighthawk-type unit is exciting, becuase it means we are likely to see Air Phase activity which, I think, will really give the game an even more distinct feel. But at this stage, I don't think it's a must-have.
Go with the Chick'n.
G&T