pretty close call but i wud suggest u can hav an effect similar to hyperconjugation acting here , making nh3+-nh3+ have a partial d.bond charecter so shorter bond length now further i say this coz N and C have comparable sizes

It has to be  N2H4

@X4 dude,

if one proton is abstracted than also a double bond, can't be formed because N doesn't have and empty orbitals to overlap, and if both the protons are abstracted one each from the N then N2H4 will be formed, so no way a double bond can be formed.

and if we c otherwise...due to the small size of N and +ve charge on both the N, there will be tremendous repuslion, due to which the bond will elongate, hence becoming very long in comparison to N2H4,

so the answer is N2H4. :)