hcbiochem
First, nitrogen tends to form 3 bonds while chlorine tends to form a single bond. One way to determine which structure is better is to calculate the formal charges on each of the two atoms (ignoring carbon because it will be the same in both cases).
Formal charge = number of valence electrons - (# of unshared electrons + 1/2 electrons in bonds)
In the first structure:
N: Formal charge = 5 - (2+1/2(6)) = 0
Cl: Formal charge = 7 - (6 + 1/2(2)) = 0
In the second structure:
N: Formal charge = 5 - (6 + 1/2(2)) = -2
Cl: Formal charge = 7 - (2+1/2(6)) = +2
The more valid Lewis structure minimizes formal charges, and so the first one is much better than the second.